Thank you for your reply!
In the meantime i found the causing problem in the Table post_timings in the database. In this table is stored, to which post the users already read in the threads. Under (for me) unknown circumstances it can happen, that the server tries to write entries in the table, that already existing.
I’ve searched in the logs and found a hint to this database table. Unfortunally i don’t know anymore, which logs i searched… 
The solution for me, was simply to delete the content of the table. All users lost their progress of reading threads, and some people had to write a new entry in existing threads just to overwrite browser cached stuff.I have a very little forum with just approx. 20 users, i don’t think, this is applicable for a forum with more users 
Greetings Ronny