Hello @Jeroen Guldemond
For version 10.6.3 , it requires only macOS 10.14 or later. Big sur should be compatible with that version.
Could you try update Microsoft Remote Desktop to Version 10.6.4 which released on 22 Apr 2021.
And here's the download link for Beta version of Version 10.6.4 (1878).
Best Regards
Karlie
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