I ran into the same issue. Did you ever find a solution?
deermichel
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Ok, it turned out that is was not a bug in AVAudioUnitSampler - running the same code on macOS gave some additional hints...
unable to obtain configuration from file:///Library/Preferences/com.apple.ViewBridge.plist due to Error Domain=NSCocoaErrorDomain Code=256 "(null)" UserInfo={NSFilePath=/Library/Preferences/com.apple.ViewBridge.plist, NSUnderlyingError=0x600003a9e610 {Error Domain=NSPOSIXErrorDomain Code=24 "Too many open files"}}
...with the second sampler, the app exceeded the soft limit for currently opened files per process (=256). The crashes afterwards were caused by failing file handles in other parts of the app. A workaround can be found at https://stackoverflow.com/a/62074374/1993349 ... done.
Addendum: Loading samples from NSDocumentDirectory instead of the bundle results in the same behavior.