Ubuntu Software aka Snap Store opens up essentially blank.
Close the Snap Store application
Open the terminal (Ctrl+Alt+T)
Type the command:
ps -ef|grep snap-store|grep -v grep…to check if snap-store process is still running
If you get anything in the output, that indicates that snap-store is still running. The output may be like:
username 24225 1871 2 21:41 ? 00:00:23 /snap/snap-store/542/usr/bin/snap-store
in that case type the command
kill (number)…where number is the first number shown in the line above, right after the username (in this case 24225)
repeat step 3 to confirm that snap-store is not running anymore
type the command:
snap-store…and watch the output for “access denied” messages.
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On Ubuntu 20.04LTS the app called “Ubuntu software” is in reality snap-store (the terminal command running it is “snap-store”, not “gnome-software”). Ubuntu 20.04 does not seem to come with gnome-software installed by default. For me, closing the app, then typing
killall snap-store
in a terminal and then re-launching the app solved the problem without a reboot.