Active Objects Error on Server Startup in Plugin Login Event Listener

hello there!
I wrote a plugin containing an event listener, to do some task when a user logs in.

Strange thing is, I get an error from within the execute method of my login event listener, trying to get active objects data:

java.lang.RuntimeException: No Hibernate Session bound to thread, and configuration does not allow creation of non-transactional one here. Listener: event:

However when I install the plugin after the confluence startup there is no problem at all.
I do not understand why LoginEvent is fired on server startup?
Can anybody give me a hint how to approach this problem?