Hi, all. I've run across this thread in the process of troubleshooting my Cura 4.8 installation (Ubuntu 20.04 to Octoprint 1.5.1 with an Anycubic Mega S)
Nothing would fix my installation, I deleted the directory at ~/home/.local/share/cura and re-initalized the program but still had the problem.
I tried a simple installation without logging in to the marketplace but still no good. I was able to do a simple non-logged in installation in Windows 10 to compare things and have it work.
Finally, I restored from an online backup from a month ago and the Ubuntu install started working again. Yay!
I'm not sure what setting got corrupted or how it still had found it after a reset but glad it was fixable and I thought I'd offer my experience to anyone else who had this happen. Backups may help.