So, there's been an interesting development -- until now, I actually only used one account to log into Windows so far, and just today I had the (belated, I admit lol) thought that I'd try to set up a separate one just to see if the issue would persist. It's... gone. I can open stuff. Must've been tied to the specific account, not system-wide. But I'd still like to be able to use my main account (it has my Microsoft account linked to it as well, the one I just set up only logs me in locally). I wonder if there could be a way to somewhat-remotely work on the problem that remains on the main account from the new one? Without reinstalling the whole OS, that is.
You can use Account Profile Fixer. It creates a new local account and migrates the data and settings from your old account to the new one.
Then just sign into the Microsoft Store to get your local account integrated with your Microsoft Account.