creating new sites doesn't work

not sure if this is about the domain mapping plugin.....
I've started a new/fresh network and now try to create new sites. This doesn't work. I get a blank screen ( see https://www.dropbox.com/s/fgx505crhwitvlb/Screenshot%202015-07-09%2022.34.19.png?dl=0 ) and lots of things go wrong afterwards.
I also noticed no database is created for the new sites.

Hint: I'm using a different prefix for the database tables.
Can you please help?