Issues with local disk & office for mac: cannot open, save, copy, or move files - error code 8003

I need some help with a recurring issue on my Mac (OS 10.15.1) which shows a number of symptoms. The main issues are:

• cannot move or copy files from local disk or onto local disk (sometimes works when copying in two steps: first step makes a "ghost" and gives error message 8003 - second step allows me to replace ghost with the real file)
• cannot open local office files (office program reacts but opens nothing; cloud files do open)
• cannot save previously opened local office files (error message: "The file ... couldn't be opened")

The issues go away very temporarily after rebooting, but come back swiftly and so I do not have a real solution.

The problem seems to be connected to my local disk and to office for mac files.