can't save or open Files with apple-apps, in specific folders

Good morning.

Since several days, my macBook Air (mid 12, MacOs 10.15.1) can't save or open (certain) files.

Every time I want to open, delete, drag'n drop a file located in ~/Desktop, ~/iCloud drive, ~/Documents, in Pages or Preview, it complains with something like

"document (document_name.suffix) couldn't be opened"

Pages couldn't read the document

I can open a file located in one of those folders with 3rd-party applications (Affinity Publisher, Coda for example).

I can copy a file and paste it into a place other than the above mentioned folders,

but I can not not drag-drop it.

As far as I can see, this problem is concerning Pages, Preview, Numbers, ..., and the 3 folders named above.

What's wrong?

How to fix this?