Openfire 4.4.1 : Disconnect after entering a chat room

Using Pidgin 2.13 as Client, Openfire 4.4.1 as Server on Windows 2012. MySQL 5.15 Database

Since a week we have the problem that users were disconnect when entering a chatroom.
Not all chatrooms do the disconnect. We foind out only those with a large history.
Even new Rooms on a new service causes a disconnect but afte reconnect they are useable.

Can’t find anything in the logs. There is enough space on the disk.

What can I do ?