read only excel Mac

Dear support/all,

since a few days I suddenly have the issue that all excel docs which I download have the status "read only" and when I open a new doc and try to save it, I get the message that I'm not authorised to save this file. The computer is mine, I do have all  the permissions to read & write etc I have even downloaded a new Office version but the problem remains. I tried all the options which excel "help" provides you with....

Can someone tell me what the problem but more important what the solution is?

Many thanks! Harrie