Word covers text with grey/glimmering rectangle

[larsrd]

I have this 70 page long MS Word document with a figure on almost every page and references to these in the text. Today I suddenly experienced that editing a specific section caused the following section to be covered by this strange rectangle as shown in the attached photo (green oval is inserted afterwards for GDPR reasons). If I try to select across the the rectangle with the mouse, I get the text which can be copied and pasted in a new document without any problems. Closing and reopening does not make the rectangle go away. In print preview everything looks normal, I see the text. Going back to a previous version and doing the same edits makes the rectangle appear in that document as well. Any ideas what is going on and how to avoid it?