Cannot edit slide master

My “Slide Master” in Impress is impervious to all attempts to edit it. I have done this thousands of times before, in Powerpoint and Impress, but now it has been switched off somehow. If I go into slide master and try to make any changes, I just get a little pop up: “Information This function cannot be run with the selected objects” I need to get this out, and I’ve spent two hours trying to find a way to tidy up what I have to send.
Please help me!

I have Office Version: 7.0.3.1 (x64)
Build ID: d7547858d014d4cf69878db179d326fc3483e082
OS: Windows 10.0 Build 19041

Try the Safe Mode, menu Help>Restart in Safe Mode....

If the error is gone, you need to reset your user profile.

Many thanks, ebot!
A little after the post I saw there was a “latest” version to download, so I downloaded Version: 7.1.0.3 (x64) and restarted with that. I opened a fresh document, then imported my “objects” (spreadsheets, maps textboxes etc.) from the old document one by one… The problem went away wuth the new one. It also opened with the wider screen format that all the powerpointers seem to use now!

A little after the post I saw there was a “latest” version to download, so I downloaded Version: 7.1.0.3 (x64) and restarted with that. I opened a fresh document, then imported my “objects” (spreadsheets, maps textboxes etc.) from the old document one by one… The problem went away wuth the new one. It also opened with the wider screen format that all the powerpointers seem to use now!