Steam version of 5.3.0 was a bit buggy so I installed this version from the official site. But now the thumbnails are gone.
When I tried to reinstall Krita shell integration plugin this error occured:“it appears that Krita 5 3 0 100 (64-bit) is installed but setup failed to locate it. This setup will now exit”
The shellex folder IS in the Krita folder. And I have uninstalled steam version of Krita because I thought it might have caused conflicts. But the thumbnails are still absent
Hello @Menalia, and welcome to the forum!
Unfortunately you ran into the most finicky to solve issue I know with Krita on Windows PCs, often it was needed to completely re-install Windows.
The uninstallation of your Steam version of Krita wasn’t necessary.
But let us begin small, perhaps you belong to the very few where it can be solved, currently I can only remember three successful attempts.
But there exist other ways to circumvent a reinstallation of Windows and nonetheless being able to view thumbnail previews, but then directly in Krita via a plugin.
You state the shell extension is installed in c:\Program Files\Krita (x64)\shellex\, is that correct?
If that is the case and the file kritashellex64.dll does exist in that folder together with the kritashellex32.dll and four other files you may be able to unregister these DLLs using a Windows command prompt with administrator rights.
To unregister both DLLs you have to open a Windows command prompt as administrator. You can do it via various ways, for instance via a click on the Windows Start button, then open the folder Windows-System and do a right mouse click RMB on Command Prompt where you then select to start it as administrator, this will open a window with a command prompt.
Alternatively you can create a link on your desktop where you enter this C:\Windows\System32\cmd.exe into the line where you are asked to browse to the file you want to link to, confirm it with the Forward-Button (can also be the OK-Button, I have no English Windows) and name it command prompt for instance. After this shortcut is created, you can open your command prompt as an administrator via a RMB click on it and the choice to start it as administrator.
Into the opening window you have to enter (as one continuous line):
regsvr32 /u "c:\Program Files\Krita (x64)\shellex\kritashellex64.dll"
Please select and copy it directly from this post and paste it into your command prompt through a RMB click on any free place in that window, this way you don’t get a typo in it, then hit your Enter-key. This was number one.
Now repeat this for the second DLL, so enter regsvr32 /u "c:\Program Files\Krita (x64)\shellex\kritashellex64.dll" as done above and hit your Enter-Key again.
If you are lucky then this will have successfully deactivated Krita’s shell extension. To test if that worked simply rename the folder c:\Program Files\Krita (x64)\shellex into c:\Program Files\Krita (x64)\shellex-old and then try to install the shell extension again.
If that does not allow you to reinstall the shell extension then please report back, and we can go through the tougher methods, which involve registry cleaning and other not so nice things. 
Michelist
Thank you Michelist for your great explanation! Sadly, it seems I’m not one of the lucky people. I have followed your method with Command prompt, but still can’t reinstall the shell extension.
Do you want to try the hard way, without any guarantee that it will work in the end, only that it can take hours to complete, or should I name you the plugins that probably could help you?
Michelist
I can go with the plugin option. It’s not that big of an issue for me (After years of using photoshop without visible thumbnails at least)
Okay, plugin number one is Imagine Board plugin, it is just recently updated and should work fine with Krita 5.3.x, if you need more information about that plugin, it has a very good wiki showcasing its abilities.
The other plugin may no longer work, but was so far my favorite for this task because it is a complete file manager as a plugin in Krita. If you give it a shot anyway is up to you. With it, you can adjust the preview sizes, the way the previews are shown and more or less anything a file manager can do, the plugin is BuliCommander, and you can read in the first post what it offers, but the version linked in the first post does no longer work, so you have to download the plugin out of this post of mine.
So far I have not tested it together with Krita 5.3 but because the most plugins (not all) work for me with Krita 5.3.0 the chance is high that it will too. But with Krita 6.0 it will no longer work, if I really begin then to try to fix this plugin from @Grum999 I can’t tell, because then I have to learn to code again, and that is something that I try to avoid for the last 3 decades right now (beside a little script juggling).
If you have further questions feel free to ask. But it is possible that I can not come to this topic back before tomorrow morning.
Michelist