My application generates following errors occasionally:

Error writing data to the connection.

Unsuccessful execution caused by a system error that precludes successful execution of subsequent statements

GDS Code: 335544727 Error Code: 407

And:

GDS Code: 335544726 Error Code: 406

And sometimes:

GDS Code: 335544332 Error Code: 12

 

These come from simple select -statement, which is executed pretty often (every few seconds).

What could cause this problem? According to error code list this e connection problem, but Firebird log doesn't show extra disconnects.

Is this caused by the client side or does this come from server?  Can the root cause be in another application accessing the database or is it the same application where the error occurs? The system has about 5 active applications accessing the database all the time.

Is there any way to debug this from server side?

Earlier I had similar problems when using same connection from multiple threads, but that should be fixed.