If you know in which component is the problem located you can experiment with this setting. But if all else fails the All/TRACE will do. The explanation of logging levels can be found on Log Levels page.
MidPoint can trace the execution of any operation using a "trace" mechanism. When enabled it records entry and exit of operations between midPoint subsystems. This is usually intended for developers but it may come handy when troubleshooting really devious problems. It can give other log records a perspective by showing where exactly the error happened and what has midPoint tries to do before that.
Trace can also be used to locate performance problems. The trace messages contain indications of how long the operation took. Therefore it can be used to roughly locate a bottleneck if some of the operations are slow.
The trace can be configured in Configuration > Logging GUI page by setting the Log subsystem entry/exit option.
Prepare to Execute an Operation
But it is a bit tricky. The server has the logfile still open. If you are on UNIX-like system it just cannot be deleted as the server will still write to the deleted file. The usual trick is to trim the file, reducing its size to zero. There are several commands that can be used in UNIX to do it. Choose any one that you like:
cp /dev/null idm.log
echo -n > idm.log