It first happend, when I changed something with the .xib-File and somehow I cannot find anything similiar (enough) on the internet that might help. And of course I did try turning it off and on again!
More details from the alert window I got:
Internal Error
File: /SourceCache/DevToolsBase/DevToolsBase-1691/pbxindex/PBXProjectIndex.m
Line: 6266
Object:
Method: _locationForNewFileRecords:
Assertion failed: headHeader && headHeader->type == PBXSymFreeBlock
Thanks for your help in advance!