M4L API in Ableton Live 12.3
The new version of Ableton Live 12.3 has a big issue in the M4L API.
The issue was reported to the Ableton support and after some test case they have been able to reproduce the issue.
There is a new public beta version available and the issue has been fixed in this beta version.
(source)
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EDIT: Ableton has fixed this issue → LINK