I found something very curious.
Before I used webparts I had an application called ?MySanbox? in the personalization application table.
Now I have two ?MySandbox? and ?/MySandbox?. Intresting because using webparts out of the box (not specifying any provider and using autocreated database) creates a an application entry called ?/MySandbox?.
I tried replacing the GUID of ?/MySandbox? to the same GUID of ?MySanbox? but got the exact same error.
Anyway maybe this information will help.