Update Manager showing: "Please switch to another Linux Mint mirror." "Your APT configuration is corrupt."

I have been getting the errors *Please switch to another Linux Mint mirror* and *Your APT configuration is corrupt* in Update Manager for the past week. I restored the default settings, so the error should not be due to the mirror. After reading a thread on forums.linuxmint.com about this issue, I ran a deep clean operation using BleachBit as suggested by a couple of users. Does this work? I tried refreshing Update Manager after the deep clean, but it's taking a long time and displaying *Updating cache: Waiting for mint-refresh-ca to exit*.