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Some or all of the below information will be requested if it isn’t supplied; for fastest response please provide as much as you can 
Nextcloud version (eg, 20.0.5): 20.0.10
Operating system and version (eg, Ubuntu 20.04): Linux infong-eu118 4.4.262-icpu-066 #2 SMP Thu Mar 18 11:50:30 UTC 2021 x86_64 GNU/Linux
Apache or nginx version (eg, Apache 2.4.25): No idea 1&1 Hosting
PHP version (eg, 7.4): 7.4
If I want to display a pdf in the browser (chrome,firefox), I get this error message:
PDF.js Version 2.4.456 (build: 228a591c)
Nachricht: Missing PDF “https://myserver/remote.php/dav/files/myname/3000%20Arbeitskreise/3200%20AK%20Kommunikation/3210%20Protokolle/2020/Protokoll%20AK%20Kom%26%C3%96A%20200113_DA.pdf”.
If a user download it, the pdf is correct and can be displayed. Working with the windows app is working too. I found the files at nextcloud/data/__groupfolders…
I think this happen since the upgrade from 20.0.8 to 20.0.10.
