When connected to a remote server via ThinLinc and I spend too much time in idle, my laptop (the client) switches to lock screen. This is expected. At this point I can enter the password and I see the server screen again. This is typically in full screen. There the mouse works (The UI responds.) but all keyboard presses are ignored. F8 is ignored too so I can't even disconnect the session. The only thing I'm aware I can do to get my keyboard back at that point is to log out and reconnect. But logging out is hassle even if I select restart session.
Both server and client are on Linux Mint 20.1.
This is unfortunately likely a bug in your screensaver. Please see bug 4776.
Which desktop environment are you running on the client? And is it Wayland or X11?
*** This bug has been marked as a duplicate of bug 4776 ***