Multi-DB - Wordpress Dashboard Error

Multi-DB is working fine.

I have set up a 16 scale DB Setup. Anything is working fine in frontend.

But when i go to Wordpress Dashboard there is a "Server Error 500". (Error log show me an Memory Error)

But this ist not the failure, when i now deactivate "Multi-DB". Wordpress Dashboard and Frontend from each Site is working fine.

Only with activated "Multi-DB" Wordpress Admin Dashboard is not working.

Any Ideas to this problem?