The most likely cause is that the onion site is offline. Contact the onion site administrator.
Even after restarting Tor with its control panel it often just doesn’t help. I need to constantly restart tor and refresh the page to get lucky.
I’m not sure if its related, but I don’t close the tor window for days usually. Never had any issue with it on regular linux
I never had such issues with tor with normal linux distros. This is directly related to the qubes with whonix.
I’ll try whonix forum if there;s no help here though