I'm experiencing the same problem indicated here (Zorin 16 Software Center), some time ago I successfully tried the proposed solution but this time after I did it twice the situation is still the same.
What additional info could I give in order to let you suggest a new solution?
Delete the directory for gnome-software located in ~/.cache/gnome-software
Then try reinstalling gnome software...
sudo apt install --reinstall gnome-software
I tried this but the problem is still there 
I hope I caught the right ".cache/gnome-software" directory.
There is only one of them, so you would have to.
Can you try:
sudo apt install --reinstall ca-certificates
killall gnome-software
If in doubt about that directory:
rm ~/.cache/gnome-software
Launch Gnome-Software or Reboot and test...
I tested and it went well but only first time.
Every time I turn on the pc the situation is the same; if I just do "killall gnome-software" and then open Software then it shows normally, but only in this way.
Do you have any added gnome-extensions?
What you could test:
sudo apt install gnome-tweaks
Launch gnome-tweak tool and move to the extensions tab. Disable all extensions.
Reboot and test gnome-software - see if it begins behaving after having removed the cache and restarting. If it does, it indicates an extension conflict and you can begin re-enabling extensions, one by one until narrowing it down on the one causing the issue.
I also wish there was an easier way of testing extensions...
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