Empty error box / can't edit Sharepoint file in desktop Mac word

Recently (past few weeks), sometimes when I open Word it displays a beige error box across the top, but it's blank. 

Today, I've been trying to open a Sharepoint file to edit in my desktop (Mac) Word, and I get that empty error box when I do, and then I can't do any editing to the doc (whether I click the error box closed or not). In fact, all of my ribbon tools are greyed out, and if I go to the File menu, I can't even create a new file. 

I've attached screen shots displaying the empty beige error box, and the greyed out ribbon tools. 

My Mac Word is up to date.

I tried the same exercise for a Powerpoint file on our Sharepoint server, and was able to open it for editing in my desktop Mac Powerpoint with no problem.