Windows 10 Store Error Code 0x80131500

There is only one account and it's clearly a Microsoft account. I never created a second one. But somehow, my email address is shown in black bold letters and uppercase and not my User Name as it should be.

I still appreciate any suggestions.

Okay, let me summarize the current situation:
Originally, as the topic name says, I tried many things to fix the Windows 10 Store Error Code 0x80131500. Among these, a first attempt of a repair upgrade didn't helped but a later try with an upgrade to Windows 10 2004 solved the problems!
However, since I tried so many things in the first place, something seems to be happened with my Windows-account. Though, all problems from my first post in this thread are solved and I can enter the Microsoft Store now, I can't download anything from it!!! (Error Code: 0x80072F8F)
My only clue is that the Windows Store-Apps Troubleshooter strangley suggests to login to a Microsoft Account (I am logged in properly).
As I wrote in post #19, I spotted something different with my account compared to my Notebook or other PCs using this account. My attemps to fix this work only temporary, as explained in the same post #19.

Any further ideas?

Okay, this is mad...

Yesterday, I managed to fix the account discrepancy. I switched my account to a local account and then back to a Microsoft account. Now my account user name is shown properly on the Welcome Screen on startup. Furthermore, the Windows Store-Apps Troubleshooter does not report the login suggestion anymore.
But it was still not possible to download anything from the store (same error code I reported in post #7). So my guess that this account discrepancy could cause the trouble was wrong...

HOWEVER

today all problems from my first post are back from the dead!!!
So the Microsoft Store denies me the access and welcomes me again with the good old Code: 0x80131500.
And the other problems are back (like no internet access with the "old" Edge Browser).

Once again, I would really appreciate if someone can help me out.
I think a very promising solution approach is still something I wrote in my first post of this thread:

"I think there must me a simple solution to the problem. Maybe my Windows configuration can't use some sockets or ports to communicate with some Microsoft services that are using these ports, sockets or whatever."

So I can browse with the old Internet Explorer and the "new" Chrome based Edge Browser, but NOT with the "old" Edge Browser.
Again: I don't really use these Browsers, but this behaviour maybe helps to sort out the problem!
The Microsoft Store and the "old" Edge Browser seem to depend on the same wrong settings (like blocked ports, sockets or whatever...) on my system.

After a new repair upgrade to Windows 10 v2004 it is again possible to enter the store but still can't download or update anything (see post #7).

However, I can install or upgrade my Apps by bypassing the Store and using .appx files directly (similar with APK files on android). Some can be installed / updated by double clicking on the file, otherwise with Windows PowerShell.

I am very happy with this situation, since I can use my PC quite "normally" now (e.g. I could finally update Windows Terminal to v1.0 and the WSL Ubuntu distribution to 20.04 LTS)

Nevertheless, any ideas to fix the Store are still appreciated