I found some references to this problem on Windows:
https://stackoverflow.com/questions/68666/clipbrd-e-cant-open-error-when-set...
https://techcommunity.microsoft.com/t5/security-compliance-and-identity/why-...
This says that a mechanism was added to allow Terminal Services to enumerate formats without opening the clipboard. I think this is the AddClipboardFormatListener function. Unfortunately, that doesn't help much with winex11 since it doesn't let us read the data.