I have verified the Online ID and Password are correct and the same on Stifel Wealth Tracker. This integration has worked for a long time without issue until the last few days.
I had the same problem a few days ago. The next morning it had started working again. I just tried it right now and the problem reappeared. I suspect Stifel might be having some ongoing connectivity issues with Quicken.
Actually, this also happened last June:
And I just tried it again right now and it was successful!
I tried it this morning and magically it was successful after failing the last 3 or 4 days. As you said, this was probably a temporary Stifel connectivity issue. Hopefully it's corrected going forward.