The problem was caused by a spurious IOResourceMatch key in the dext IOKitPersonalities, left over from the initial Xcode project template. After deleting the key all seems ok (as you would expect!).

The entire entitlements application system appears to be broken. I applied for entitlements 8 months ago. After six months they were granted, but the settings on my developer account were incorrectly configured and they can only be partially used. Follow-ups to the grant request by email are seemingly ignored and neither DTS nor general developer support appear to be able to help. This is an appalling situation: I have customers waiting to release software and we can not simply because of Apple's non-functional internal procedures. Had we decided to stick with kexts we would at least be able to ship. Is there some way that these problems can be escalated to senior management, and adequate resource be put in place to avoid this disaster?

Ok thanks - we will confirm this when the final 11.3 release is ready!

I think that I found the problem: if anything fails in the init() method of the user client object, rather than propagating the error back from Create(), DriverKit crashes. Moral: UserClient init() methods must, at least for now, always succeed.

I should have added that I am using Xcode 16.3. I have successfully made driverkit distribution releases previously, but likely not since the requirement for iOS was introduced.