Unable to trace "Arithmetic overflow error converting numeric to data type numeric" error in profiler
Using the follow link you can set everything up to track the exception This link

In short: Set up a blank trace, add the following events : 


Exception under Errors and Warnings
RPC: Completed under Stored Procedures
RPC: Starting under Stored Procedures
SQL: BatchCompleted under TSQL
SQL: BatchStarting under TSQL


From there on, you can see the error popping up in the trace, the query above the error will be your query that's giving the error.

If that's not giving you the exception, i'd debug the .NET code to get the SQL code that's being executed, if you have access to the source code.