Any update here as I'm having this issue after updating to the latest version 2019.2? I had the issue happen a few weeks ago and it magically fixed itself somehow. I've been a moneydance user for years, but reliability has slipped recently.
And now you said that it was working in-between your OP and now. I don't think that your update did anything to break things. This sounds like it's most likely a 5/3 problem, especially since it's intermittent and there has been no word from IK as to any changes on their end.
BTW there is a newer preview version if you wanted to try that next:
I think the "magic" had more to do with me resetting the connections versus upgrading and all that. Essentially I upgraded to 2019 it didn't work so I reverted back and fixed the connections and it worked until I upgraded to 2019.2 yesterday. I struggle to think this is on the 5/3 side. MD support has been slow to respond at best. I've tried to revert back but the connection still gives me the login error. In the end I'm spending way to much time trying to figure out the issue.
Thanks for the added information Shane. Unless they reply in private, there has been absolutely no response from IK (that I have seen) in the several links related to this matter. It is quite perplexing.
I am also having the same issue. Out of curiosity I tried to use Quicken to see if the problem existed there as well. Quicken 2019 also cannot connect to Fifth Third bank. From conversations with Moneydance support, Quicken support, and Fifth Third bank it appears to be related to a server change at fifth third bank. However, none of them are offering any time line for a resolution, only "our developers are working on it"
If anybody has any positive response, please post!