Thanks for the response! Your assumption that it most probably is a threading problem made me invest 2 more days into investigating thread access to all my CBL objects.
I was using the shared CBLManager instance (normally used from main thread in my app) from a background thread. The enclosing method should have been called from main thread but wasn’t because of a bug some steps higher in the call hierarchy.
Activating lots of log messages (DatabaseVerbose + QueryVerbose + ViewVerbose) didn’t take me much closer to the solution because the “thread violation” occurred very long before the first effects. (and the code where the effects occurred was not violating the threading rules, indeed).
The key to finding the problem was adding a quick and dirty checkThread() method to CBLManager, CBLDatabase, CBLView and CBLQuery (which checks, if the calling thread is equal to the thread from which the object was generated and raises a NSException if not) and calling that method within all main methods of these classes.
So, maybe such a “thread-safety enforcement mode” would help more people facing threading (or just inexplicable) problems like me? What do you think?