Word problem and footnotes highlighted in black

Since 2 days, the references in my footnotes are highlighted in black (not in gray as it sometimes happens). When that happens, I have to close my Word document and reopen it to get everything back to normal. I think it’s a problem with Word but can’t find a solution.
I use Office 365, and Word, Excel … are up to date.
Anyone know this problem and its resolution?

Thank you