Hey sorry for the issues you are facing - sounds frustrating
Here's some tips I've collected
Try resetting your preferences: The easiest way to reset everything is to delete the InDesign preferences: as soon as InDesign starts to boot hold down Shift, Alt, Ctrl (and Apple/Cmd if working on a Macintosh). A dialog will appear asking to delete the InDesign Preferences, select 'Yes' on this dialog. Further info and instructions here if needed: https://helpx.adobe.com/in/indesign/kb/indesign-preferences-support-file-locations.html
Thank you for your reply. Deleting the preferences didn't work unfortunately. I work with an Indesign 2021 refurbished licence, not CC, so the Creative Cloud Cleaner will probably not work? Opening a new document wasn't an issue at the beginning. If I remember correctly it started when I opened an old file that caused a problem.