After some uptime (couple hours?), it may happen that Thunar cannot open anymore until the next boot. File Manager in the Start Menu has no effect and running thunar in Terminal gets "Failed to register: Timeout reached".
There is no unusual behavior around the time it first happens.
Syslog did show that xfce4-notifyd service crashed at that time. Re-logging in brought it back up, but the problem persisted.
Code:
Oct  2 01:41:42 IB-PC xfce4-notifyd[1808]: xfce4-notifyd: Fatal IO error 11 (Resource temporarily unavailable) on X server :0.
Oct  2 01:41:42 IB-PC systemd[1656]: xfce4-notifyd.service: Main process exited, code=exited, status=1/FAILURE
Oct  2 01:41:42 IB-PC systemd[1656]: xfce4-notifyd.service: Failed with result 'exit-code'.

● xfce4-notifyd.service - XFCE notifications service
     Loaded: loaded (/usr/lib/systemd/user/xfce4-notifyd.service; static; vendor preset: enabled)
     Active: active (running) since Fri 2020-10-02 01:54:46 PDT; 15min ago
There are no more clues around this time in any of the log files.