Get SQL query in logon trigger
As Brent Ozar noted in his answer, you are attempting to use a Logon trigger for a purpose for which it was not designed.  Logon triggers are typically used to either capture details about who is logging onto the server, or to deny particular combinations of users/machines etc from connecting.

Luckily, Extended Events offers a mechanism for capturing T-SQL in memory that, when configured correctly, can be fairly light on system resources.

I've used something like the following code in the past to capture T-SQL from a particular user or machine:

IF EXISTS 
(
    SELECT 1 
    FROM sys.server_event_sessions dxs 
    WHERE dxs.name = 'queries'
)
BEGIN
    IF EXISTS (
        SELECT 1 
        FROM sys.dm_xe_sessions dxs 
        WHERE dxs.name = 'queries'
    )
    BEGIN
        ALTER EVENT SESSION queries 
        ON SERVER 
        STATE = STOP;
    END
    DROP EVENT SESSION queries 
    ON SERVER;
END

CREATE EVENT SESSION queries ON SERVER 
ADD EVENT sqlserver.sql_statement_starting
(
    ACTION 
    (
        package0.collect_system_time
        , package0.event_sequence /* SQL Server 2012+ */
        , sqlserver.client_app_name
        , sqlserver.client_hostname
        , sqlserver.database_name /* SQL Server 2012+ */
        , sqlserver.plan_handle
        , sqlserver.sql_text
        , sqlserver.username
        , sqlserver.request_id
        , sqlserver.session_id
    )
    WHERE sqlserver.username = N'some_user_name'
        AND sqlserver.database_id = 6 /* track a specific database only */
        AND sqlserver.client_hostname &lt;&gt; 'excluded_host_name'
) 
, ADD EVENT sqlserver.sql_statement_completed
(
    ACTION 
    (
        package0.collect_system_time
        , package0.event_sequence /* SQL Server 2012+ */
        , sqlserver.client_app_name
        , sqlserver.client_hostname
        , sqlserver.database_name /* SQL Server 2012+ */
        , sqlserver.plan_handle
        , sqlserver.sql_text
        , sqlserver.username
        , sqlserver.request_id
        , sqlserver.session_id
    )
    WHERE sqlserver.username = N'some_user_name'
        AND sqlserver.database_id = 6 
        AND sqlserver.client_hostname &lt;&gt; 'excluded_host_name'
) 
, ADD EVENT sqlserver.error_reported 
(
    ACTION 
    (
        package0.collect_system_time
        , package0.event_sequence /* SQL Server 2012+ */
        , sqlserver.client_app_name
        , sqlserver.client_hostname
        , sqlserver.database_name /* SQL Server 2012+ */
        , sqlserver.plan_handle
        , sqlserver.sql_text
        , sqlserver.username
        , sqlserver.request_id
        , sqlserver.session_id
    )
    WHERE sqlserver.username = N'some_user_name'
        AND sqlserver.database_id = 6 
        AND sqlserver.client_hostname &lt;&gt; 'excluded_host_name'
        /* fluff errors below - for SQL Server 2008 R2, use "error" instead of "error_number" */
        AND error_number &lt;&gt; 5703 /* Changed language setting to %.*ls. */
        AND error_number &lt;&gt; 5701 /* Changed database context to '%.*ls'. */
        AND error_number &lt;&gt; 2528 /* DBCC execution completed. If DBCC printed error messages, contact 
                            your system administrator. */
        AND error_number &lt;&gt; 7969 /* No active open transactions. */
        AND error_number &lt;&gt; 4035 /* Processed %I64d pages for database '%ls', file '%ls' on file %d. */
        AND error_number &lt;&gt; 18265/* Log was backed up. Database: %s, creation date(time): %s(%s), 
                first LSN: %s, last LSN: %s, number of dump devices: %d, device information: (%s). 
                This is an informational message only. No user action is required. */
        AND error_number &lt;&gt; 3014 /* %hs successfully processed %I64d pages in %d.%03d seconds (%d.%03d MB/sec). */
        AND error_number &lt;&gt; 14570/* (Job outcome) */
        AND error_number &lt;&gt; 8153 /* Warning: Null value is eliminated by an aggregate or other SET operation. */
)
ADD TARGET package0.ring_buffer
(
    SET max_memory = 1024
)
/* add or remove the below target as required. */
, ADD TARGET package0.asynchronous_file_target
(
    SET filename = 'C:\temp\queries_xe_target.xel'
        , max_file_size = 10        /* max size in MB */
        , max_rollover_files = 10
        , increment = 1             /* file growth increment in MB */
)
/* Don't start this Extended Events session automatically when the server starts */
WITH 
(
    STARTUP_STATE = OFF                                 /* Extended Event Session will NOT be automatically 
                                                            started at server startup */
    , TRACK_CAUSALITY = ON
    , MAX_MEMORY = 5MB                                  /* buffer size to use */
    , EVENT_RETENTION_MODE = ALLOW_SINGLE_EVENT_LOSS    /* ALLOW_MULTIPLE_EVENT_LOSS or NO_EVENT_LOSS */
    , MAX_DISPATCH_LATENCY = 15 SECONDS                 /* maximum number of seconds until buffer contents 
                                                            are written to the target */
    , MEMORY_PARTITION_MODE = PER_NODE                  /* NONE, PER_NODE, PER_CPU */
);
GO


Use this to start the session as required:

IF EXISTS 
(
    SELECT 1 
    FROM sys.server_event_sessions dxs 
    WHERE dxs.name = 'queries'
)
BEGIN
    ALTER EVENT SESSION queries ON SERVER STATE = START;
END


This shows the session target details:

SELECT SessionName = xe.name
    , TargetName = xet.target_name
    , EventData = CONVERT(xml, xet.target_data)
FROM sys.dm_xe_session_targets AS xet
    INNER JOIN sys.dm_xe_sessions AS xe ON (xe.address = xet.event_session_address)
WHERE xe.name = 'queries';


Use this to see the results from the ring buffer:    

DECLARE @xml XML;

SELECT TOP(1) @xml = CONVERT(xml, xet.target_data)
FROM sys.dm_xe_session_targets AS xet
    INNER JOIN sys.dm_xe_sessions AS xe ON (xe.address = xet.event_session_address)
WHERE xe.name = 'queries'
    AND xet.target_name = 'ring_buffer';

IF OBJECT_ID('tempdb..#xmlResults') IS NOT NULL
DROP TABLE #xmlResults;

CREATE TABLE #xmlResults
(
    RowNum INT NOT NULL
        PRIMARY KEY CLUSTERED
        IDENTITY(1,1)
    , xeXML XML NOT NULL
);

INSERT INTO #xmlResults (xeXML)
SELECT xm.s.query('.')
FROM @xml.nodes('/RingBufferTarget/event') AS xm(s)
OPTION (Optimize FOR (@xml = Null)); -- immensely improves performance in SQL Server 2008

SELECT t.EventName
    , DateStamp = DATEADD(HOUR, -6, t.EventDateStamp)
    , DatabaseName = d.name
    , t.ErrorNumber
    , t.ErrorSeverity
    , t.ErrorState
    , t.ErrorMessage
    , t.CollectSystemTime
    , t.ClientAppName
    , t.ClientHostName
    , t.PlanHandle
    , t.SqlText
    , t.UserName
FROM (
        SELECT EventName =          xeXML.value('(event/@name)[1]','varchar(500)')
            , EventDateStamp =      xeXML.value('(event/@timestamp)[1]','datetime')
            , DatabaseID =          xeXML.value('(event/data[(@name)[1] eq "source_database_id"]/value/text())[1]','varchar(255)')
            , ErrorNumber =         xeXML.value('(event/data[(@name)[1] eq "error"]/value/text())[1]','varchar(255)')
            , ErrorSeverity =       xeXML.value('(event/data[(@name)[1] eq "severity"]/value/text())[1]','varchar(255)')
            , ErrorState =          xeXML.value('(event/data[(@name)[1] eq "state"]/value/text())[1]','varchar(255)')
            , ErrorMessage =        xeXML.value('(event/data[(@name)[1] eq "message"]/value/text())[1]','varchar(255)')
            , CollectSystemTime =   xeXML.value('(event/action[(@name)[1] eq "collect_system_time"]/text/text())[1]','varchar(255)')
            , ClientAppName =       xeXML.value('(event/action[(@name)[1] eq "client_app_name"]/value/text())[1]','varchar(255)')
            , ClientHostName =      xeXML.value('(event/action[(@name)[1] eq "client_hostname"]/value/text())[1]','varchar(255)')
            , PlanHandle =          CONVERT(xml, xeXML.value('(event/action[(@name)[1] eq "plan_handle"]/value/text())[1]','varchar(255)')).value('(plan/@handle)[1]', 'varchar(255)')
            , SqlText =             xeXML.value('(event/action[(@name)[1] eq "sql_text"]/value/text())[1]','nvarchar(max)')
            , UserName =            xeXML.value('(event/action[(@name)[1] eq "username"]/value/text())[1]','varchar(128)')
        FROM #xmlResults xm
    ) t
    LEFT JOIN sys.databases d ON t.DatabaseID = d.database_id
WHERE t.UserName NOT IN (
      'user_1'  COLLATE SQL_Latin1_General_CP1_CI_AS
    , 'user_2'  COLLATE SQL_Latin1_General_CP1_CI_AS
    , 'user_3'  COLLATE SQL_Latin1_General_CP1_CI_AS
    ) 
ORDER BY t.UserName
    , t.EventDateStamp;

SELECT *
FROM #xmlResults