I've been getting the same thing, but it's totally random. One day it starts throwing a CC-800 with one account. Keep doing the "fix it" (re-add) and it says it did it and keeps throwing the error. Then a week later, a different account throws the error. Fixit doesn't fix it. Now it's doing it on Citibank. The only "fix" to fix the broken accounts is to deactivate them and then reactivate. Now as to why Quicken keeps thinking that accounts have been deleted it beyond me. So far it's done it with my local credit union - SEFCU, my local bank - TrustCo, and now Citibank.
Yes, followed the steps, repeatedly. The error has come back again for Citibank, although it does appear it actually downloads transactions. Given that other users ALSO have this situation I can only conclude it's a bug with Quicken.