I solved this issue by disabling Persistent bitmap caching in mstsc options for my VM:

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After doing this, I haven't been experiencing any issues for several month.


For me, disabling 'Persistent bitmap caching' as recommended here worked. However, this reduces performance.

Instead, deleting the contents of C:\Users\\AppData\Local\Microsoft\Terminal Server Client\Cache and connecting again worked. This way I was able to keep 'Persistent bitmap caching' turned on, but not have any black boxes.