Don't know what I did exactly, but now it works, either with standard Mach service, or with custom...
Added App Groups entitlement to the App and Extension, also to Provisioning Profile (in spite of the App is not Sandboxed),
Changed certificate from Apple Development to Developer ID,
Added exported object from the Extension side and called it's method from the App side.
Solved.
MeirS
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The same thing also for me.
In our case, the reason of the issue was preventing GET_TASK operation for the crashed process in Endpoint Security system extension. coreservicesd needs it in Monterey when the process calls NSFileManager's methods.
The solution was muting the coreservicesd for es_client intercepting ES_EVENT_TYPE_AUTH_GET_TASK.