this isn't a supported file type - Images, movies, pdf files, voice messages cannot be opened after a while

A friend has a Nokia 7.2 (updated to Android 11) which has a problem opening images, movies, pdf files. When she makes or receives a photo or movie, she can open it normally, but after a while she gets the following message: this isn't a supported file type. So it looks like an app scans the system for media, then does something to it so it can't be viewed again. She did a factory reset, but the problem persists. Do you have any ideas how this works? Is this a virus? Did it infect the factory-image? How can this be fixed?