When I click on a document, I get a small box with a red X on top and the word “file” on the bottom. Can’t open a new document either. Why?
Why
Just open LibreOffice and use File -> Open instead of relying to click on icons. That’s the only way to tell (decide), whether the icons have a buggy configuration or the problem is with LibreOffice.
There might be two issues here. The red X might be a file flagged as not being synched to Dropbox, GoogleDrive, OneDrive, etc.
You can’t open a new document either. Can you open LibreOffice?
It could be an aggressive setting in your anti-virus or operating system.