Again today - just booted, started Terminator (which had been working earlier that day) then it started. When I tried CTRL-ALT-F2 nothing happened except the mouse pointer disappeared and nothing responded at all. But then with CTRL-ALT-F7 everything started working normally again, I could then switch TTY as normal, and it's still fine a couple of hours later. The X session refreshed or something? I switched from XFCE to MATE now, to see if it happens in future.
https://pastebin.com/KX2RT38E
$ ag "(EE)" Xorg.0.log
[ 10.413] Current Operating System: Linux ax 5.9.14-zen1-1-zen #1 ZEN SMP PREEMPT Sat, 12 Dec 2020 22:20:32 +0000 x86_64
(WW) warning, (EE) error, (NI) not implemented, (??) unknown.
[ 10.424] (EE) Error systemd-logind returned paused fd for drm node
[ 10.498] (EE) Failed to load module "nv" (module does not exist, 0)
[ 10.662] (II) Initializing extension MIT-SCREEN-SAVER
[ 11.825] (EE) systemd-logind: failed to take device /dev/dri/card0: Device or resource busy
[ 11.825] (EE) /dev/dri/card0: failed to set DRM interface version 1.4: Permission denied
[ 46.003] (EE) event12 - AlpsPS/2 ALPS DualPoint Stick: client bug: event processing lagging behind by 17ms, your system is too slow
[ 67.997] (EE) client bug: timer event13 trackpoint: scheduled expiry is in the past (-357ms), your system is too slow
[ 67.997] (EE) client bug: timer event13 trackpoint: scheduled expiry is in the past (-71ms), your system is too slow
[ 67.997] (EE) client bug: timer event13 trackpoint: scheduled expiry is in the past (-60ms), your system is too slow
[ 67.997] (EE) client bug: timer event13 trackpoint: scheduled expiry is in the past (-48ms), your system is too slow
[ 67.997] (EE) client bug: timer event13 trackpoint: scheduled expiry is in the past (-35ms), your system is too slow
[ 67.997] (EE) client bug: timer event13 trackpoint: scheduled expiry is in the past (-22ms), your system is too slow
[ 67.998] (EE) client bug: timer event13 trackpoint: scheduled expiry is in the past (-10ms), your system is too slow
[ 84.034] (EE) event13 - AlpsPS/2 ALPS DualPoint TouchPad: client bug: event processing lagging behind by 1250ms, your system is too slow
[ 123.873] (EE) client bug: timer event12 debounce: scheduled expiry is in the past (-513ms), your system is too slow
[ 123.873] (EE) client bug: timer event12 debounce: scheduled expiry is in the past (-399ms), your system is too slow
[ 123.873] (EE) client bug: timer event12 debounce short: scheduled expiry is in the past (-412ms), your system is too slow
[ 172.938] (EE) client bug: timer event13 middlebutton: scheduled expiry is in the past (-1937ms), your system is too slow
[ 172.938] (EE) client bug: timer event13 middlebutton: scheduled expiry is in the past (-1709ms), your system is too slow
[ 178.949] (EE) event13 - AlpsPS/2 ALPS DualPoint TouchPad: client bug: event processing lagging behind by 1973ms, your system is too slow
[ 235.376] (EE) client bug: timer event12 debounce: scheduled expiry is in the past (-22ms), your system is too slow
[ 257.387] (EE) event13 - AlpsPS/2 ALPS DualPoint TouchPad: client bug: event processing lagging behind by 402ms, your system is too slow