Copy text (Ctrl-C or context menu) not working

This is actually the same in Firefox, and both only since today, working fine before: Typing Ctrl-C does nothing, Ctrl-X deletes whatever is selected, but it does not end up in the clipboard. In fact, if the clipboard contains something, it will be removed (Win-V still shows it, but it can no longer be pasted). When copying from somewhere else, pasting with Ctrl-V works. AFAICT, no issues with any other programs. Latest versions of FF (151.0.2) and Thunderbird (151.0.1), on Windows 10. What could be wrong? Thanks, Ralf