Can't save: "The file may be read-only, or another user may have it open. Please save the document with a different name or in a different folder."

I've seen this issue all over the internet when I've searched for an answer. And it looks like it has been around for years. The only solution I've seen is to close the window pane in Windows. But what about for Mac users?

I just bought a new macbook pro. I had a 2011 macbook pro previously, with Acrobat Pro X. I backed up my computer in time machine, started up the new computer, and restored the old computer's data to the new one. Everything is the same and works fine except that I cannot save PDFs when I edit them. The only workaround I've found is to open a new folder, which  I can "save as" to the new location, but then I can't save future edits there unless I send them to yet another new folder.

The error message is: "The file may be read-only, or another user may have it open. Please save the document with a different name or in a different folder."

This is driving me crazy. Someone please help.