I'm pretty sure this one isn't Quicken's fault, but rather Synchrony Bank.
I have the Amazon Store card (ie: Synchrony Amazon), the Synchrony Mastercard (formerly Rakuten/eBates card), Lowe's (Synchrony bank)
The fun parts:
Synchrony Bank has merged all their sites into an aggregate. So if you login (web browser) to the Amazon link, it'll show you ALL your Synchrony cards. Likewise the Lowe's one, etc.
The problem:
Quicken's integration doesn't seem to yet be aware of all of this (or possibly the way Synchrony has setup for Quicken to access).
So for example, if I try to setup the Synchrony Mastercard (which the “official” URL is: synchronymastercard.com) within Quicken (quicken has it as: mastercard.syf.com), Quicken fails to connect. If I choose the Synchrony Financial Mastercard (www.synchronycredit.com), it also fails to connect with Quicken. (but ironically if you use that URL in a web browser, it'll redirect you to the synchromastercard.com site).
The only one's that work, that I've found is the:
Lowe's Commercial Card
and
Amazon.com Store Card
Of course, if you choose either option, it'll find all your cards and it seems to mess things up (they will randomly start giving you the CC-400 or something error I think where you keep going through the “fix it” loop). Especially if you “accidentally” setup say, your Lowe's card for the Lowes.com website and have the Amazon.com card using the Synchrony website.
My last attempt will be to deactivate them all, and re-enable online via ONE (ie: Choose Amazon.com and when it finds all the accounts, link to existing so they all have the same, yet wrong, URL). And see if the random disconnects keep happening.
I'll try to snag the error message if/when it happens again.
Perhaps Quicken should update their list of online accounts so they point to the correct URL's/locations?