I got in touch with the customer and looked at the crash reports.
The crash is due to a file called libpianoprocessor.dylib and there are multiple versions of that file, all with different sizes (sigh)
They are used by Arturia Lab 4 and 5 and by PianoV3
We disabled those plugins in the plugin manager and that solved the problem (at least GP no longer crashes - the customer will need to use different plugins for those sounds)