<<that's the question, that I have: How to find out what causes the deadlock scans. I have thousands of transactions per minutes,
doing all possible kind of things. What to look for? Where?>>

Thats what a lock print is for - when it "hangs the next time" get a lock print
fb_lock_print -a -d database > lock.txt
Once you have it - you can analyse it.

https://www.ibphoenix.com/resources/documents/how_to/doc_402
http://www.ibphoenix.com/files/conf2011/Lock-Print-Yemanov-2011.pdf

Regards
Paul