Hello everyone
I now have this problem as well. It happen to me after using "Timeshift" to restore my system from the previous day's snapshot. Note that I have activated the Timeshift option to "include hidden files from my home directory" in the snapshot.
Update
I set up a new user and logged into that account. The "Welcome" and "Software Boutique" worked. I installed a program from the Software Boutique and logged back into the first "broken" account. The newly installed program was there ready to use.
Note also that I had two snap programs besides "Welcome" and "Software Boutique". Rolling-back the system from the previous day's snapshot had no effect on them.