Edit: Last night, I used the "Fix MergeList problems" option in the maintenance tab of software sources and at least for now, it seems to be working. So I probably wont need help with this anymore, hopefully.
Every hour or two, the Update Manager keeps giving me an error message saying that my APT configuration is corrupt and that I should switch to another Linux Mint mirror. It usually goes away when I do a manual refresh but it just keeps coming back. I have also tried switching to a different mirror but I get the same error. It also tells me to run apt-get update but even if I add "sudo" it just gives me an error message saying to run apt-get update.
It happened again so I'm adding screenshots in case they help. The first is the initial error, then it's the error I get when I try to change the repository and the third is the error I get when I try running the command it suggested:
I might try that later if the issue comes back up. Last night, I used the "Fix MergeList problems" option in the maintenance tab of software sources and that seems to have fixed it. I did at the time think it was an issue with my HDD but disks wont let me check the filesystem because it can't unmount a filesystem that is in use.
Boot up from a USB drive that's what I did. I was just about to add the merge list in the update manager as well. Keep me posted if it didn't work. Best of luck.