Microsoft Store and various apps don't work in Windows 10 version 1903

"(1) You found a work-around? To do non-trivial edits first? Good going, then."

I can't figure out what you mean.  If I do any non-trivial edits, PDFill deletes the .pfl file instead of overwriting, and it can't write the new .pdf.  PDFill still works on a different PC which also runs Windows 10 1903 with the latest updates, so the workaround is to use the other PC.

"(2) I like the virtues I itemized earlier on Windows Defender. I would leave it as the Real-Time scanner."

I did.  When Malware Bytes was installed and running, it did not interfere with Windows Defender.  On other PCs also, Malware Bytes does not interfere with Windows Defender.

You say that EICAR experiments prove that Windows Defender works well.  In fact EICAR experiments prove that Windows Defender catches EICAR but do not prove that Windows Defender catches WannaCry.  But this is irrelevant because I never disabled Windows Defender anyway, and it catches what it catches.  It certainly does not explain why the Store, Calculator, and maybe other Store apps, and now the non-Store app PDFill, stopped working.  By the way I read that Notepad is going to be changed into a Store app, sigh.

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Meanwhile, Windows Update has not tried to update the Store and some Store apps since 2019-08-11.  They still don't work though.

(1) You found a work-around? To do non-trivial edits first? Good going, then.

(2) I like the virtues I itemized earlier on Windows Defender. I would leave it as the Real-Time scanner. Maybe install Malwarebytes - but do not activate its Real-Time scanning. Just run it occasionally On-Demand. And send them a check after you've done it twelve times. IIRC, it found a bunch of PUPs first time I ran it, but nothing ever after. I uninstalled it after my 11th run. Defender catches/blocks all 3 items here...

http://rexswain.com/eicar.html

EICAR Test Virus

That's in Edge. In "Defender, Virus & Threat Protection, Protection History" I see (for the 2 .zip's that weren't blocked)...

Click "learn more" to see it really is a test. This site opens in Edge...

https://www.microsoft.com/en-us/wdsi/threats/malware-encyclopedia-description?name=Virus%3aDOS%2fEICAR_Test_File&threatid=2147519003

Virus:DOS/EICAR_Test_File

But why does it say "remediation  incomplete"? Well, later it says "might not". Anyway, the Downloads folder has no EICAR file at all in it - so, I guess it is completely gone. A Defender quick scan finds nothing. At first, under "Actions", there were choices to remove, quarantine, & allow on device. I chose "remove" - & now the only choice left is "allow" - why is it left? I'm thinking, if I click "allow", it will put that file into the exclusions folder.

So - that proves Windows Defender works well. But how do I get those items out of Protection History now? I don't like extraneous items to be lying around.

Edit: Now I've run a full Defender scan, & it found EICAR junk in two folders that belong to Edge, which look like TIFs (Temporary Internet Files) & a temporary download folder....

I chose "remove", & they were deleted. (I checked before & after.) I guess those were harmless anyway (even in case they were real). Here is what the above looks like afterward...

Here is what TempState looked like before the remove...

I still think Defender is fine - but a tad sloppy. How do I get rid of this extraneous matter in Protection History? A reboot didn't do it. And why do those other two still say "remediation incomplete"?

Edit 8/19: I deleted the Microsoft-Windows-Windows Defender log in Event Viewer in an attempt to get rid of that Protection History, but the history did not go. There were two event ID 1117 that said a quarantine failed because the virus file could not be found. I don't recall asking for a quarantine, but I guess that explains the "remediation incomplete" on those two.

It is weird that PDFill is able to do all of the editing operations that I do, but when it comes time to save the project file (.pfl) or pdf file then its success or failure depends on whether I had done any non-trivial edits before that.

Many years ago I read that Exel does a silly thing when overwriting a spreadsheet, first deleting the old file and then writing the new one (which might fail, leaving the file deleted) because of the way Windows used to operate many years ago.  It would not surprise me if PDFill copied that behaviour, which might have made sense many years ago, though I wouldn't do things that way in any program I write.

Maybe I should reinstall Malware Bytes on the messed up PC  ^_^

You did everything right trying to fix PDFill. I can think of little more, except to see whether it can be updated. Does the other PC have the same version? There is something in the write-up...

https://support.microsoft.com/en-us/help/4512508

August 13, 2019—KB4512508 (OS Build 18362.295)

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But you don't report getting that error message. And, anyway, why would it apply to one computer with the app & not the other?

I guess, go ahead & uninstall the update. It will return - maybe better.

Yesterday's quality update made things worse.  All the non-Store applications that I have were working until yesterday, but one broke.

2 days ago I used PDFill normally.  Today PDFill is usually deleting the .pfl file that it's supposed to overwrite, and failing to create a file for the edited .pdf file.  I experimented doing a very simple edit and not adding pages, and PDFill still worked, but it now fails when used for a normal amount of editing.

I tried repairing PDFill, and then uninstalling and reinstalling, but it's still the same.

I'm half-inclined to uninstall yesterday's quality update.

Though on my other PC where Store still works, PDFill still works too, even after yesterday's quality update.