Tor Browser version check url has been changed by The Tor Project (Move RecommendedTBBVersions from check.torproject.org to www.torproject.org (#8940) · Issues · Legacy / Trac · GitLab).
Please run these two commands.
sudo sed -i "s#https://check.torproject.org/RecommendedTBBVersions#https://www.torproject.org/projects/torbrowser/RecommendedTBBVersions#g" /usr/lib/whonix/whonixcheck/65_check-torbrowser ; echo $?
sudo sed -i "s#https://check.torproject.org/RecommendedTBBVersions#https://www.torproject.org/projects/torbrowser/RecommendedTBBVersions#g" /usr/bin/torbrowser ; echo $?
Then check if whonixcheck and Whonix’s Tor Browser updater are still working for you.
If this works, I plan to write a blog post about this.