I could not find a suitable solution for me. The only solution i had was creating a SQL-Job that cleans the database on regular bases. But this is not a real solution it is more like minimize the effect instead of really solve the cause of the problem.
I now decided to reinstall vCenter. After i read, that the vCenter Appliance has its own embedded databases for SSO and vCenter, i decided to use the appliance because i hope that the embedded DB will not have this issues... We will see...
I deployed the OVA Template and after some problems on adding to the domain all runs fine now and vCenter is stable for more than a week now. I'm happy with that! I let you guys know when i run into the same problem with the appliance.
As far as i can judge it - use vCenter Appliance!
Thanks alot for your help!