screenshot save hangs - 100% CPU usage

Hi! I click the screenshot icon in the top panel, then select "active window" with 1s delay, OK, then I can see the screenshot preview, I select save, click OK and then... nothing happens, except that xfce4-screenshooter is suddenly consuming 100% of CPU (1 core out of 8). I wait some 5-15 seconds, then finally I kill it using xfce4's force close.

I can successfully take a screenshot using the same procedure but a different user within the same Xfce session (su inside xfce4-terminal with session optional pam_xauth.so) (I use my own pam_supair to switch the user). No problem with that. For my main user it used to work for weeks in the current Xfce4 session, but then suddenly it's not working anymore.

What's wrong?

This is Xfce version 4.12 as distributed by Debian.

$ cat /etc/debian_version 
10.13

Thanks