Error after SQL-Server Update: The prepared statement with handle xyz was not found
You can potentially find the error message through Extended Events. Create and start the following session:

CREATE EVENT SESSION FailedQueries
ON SERVER
 ADD EVENT sqlserver.error_reported
 (
   ACTION 
   (
     sqlserver.client_app_name,
     sqlserver.client_hostname,
     sqlserver.nt_username,
     sqlserver.sql_text
    )
  )
  ADD TARGET package0.asynchronous_file_target
  (
    SET FILENAME = N'C:\temp\FailedQueries.xel',
    METADATAFILE = N'C:\temp\FailedQueries.xem'
  );
GO

ALTER EVENT SESSION FailedQueries ON SERVER STATE = START;
GO


To make sure it is working, run a query that will definitely raise an exception:

SELECT 1/0 FROM sys.objects;


Check the session to make sure this event populated:

;WITH event_data AS 
(
  SELECT data = CONVERT(XML, event_data)
    FROM sys.fn_xe_file_target_read_file(
   'C:\temp\FailedQueries*.xel', 
   'C:\temp\FailedQueries*.xem', 
   NULL, NULL
 )
),
results AS
(
  SELECT 
    [host]      = data.value('(event/action[@name="client_hostname"]/value)[1]','nvarchar(4000)'),
    [app]       = data.value('(event/action[@name="client_app_name"]/value)[1]','nvarchar(4000)'),
    [date/time] = data.value('(event/@timestamp)[1]','datetime2'),
    [query]     = data.value('(event/action[@name="sql_text"]/value)[1]', 'nvarchar(max)'),
    [error]     = data.value('(event/data[@name="error_number"]/value)[1]','int'),
    [state]     = data.value('(event/data[@name="state"]/value)[1]','tinyint'),
    [message]   = data.value('(event/data[@name="message"]/value)[1]','nvarchar(250)')
  FROM event_data
)
SELECT * FROM results
WHERE error NOT IN (5701,5702,5703)
ORDER BY [date/time] DESC;


Then you can periodically check the output of this query (maybe adding a filter like AND [message] LIKE N'%prepared statement%'). Hopefully that gives you some additional clues.