Can't backup anymore on my Windows computer.

hswcb

It says "unable to open source file". I don't know what that means.

miklk

That normally means your data file is being backed up by a program like Dropbox or Microsoft One Drive (newer computer sometimes set up by default)

You can pause them and do backup.

Quicken suggests data file should be in default Documents/Quicken to avoid these kind of problems and possible corruption.

If above us not the case, anti virus can sometimes cause that too

hswcb

First - I don't know how to "pause" Dropbox or One Drive. I tried to back up to my computer's Documents/Quicken file and still got the same message: "Unable to Open Source File"