Atlassian knows for sure, since it is their message.
But it will be hard for you to get any help from them on this portal. The best would be to contact them directly and ask for assistance (raise DEVHELP ticket).
By chance we encountered a similar error dialog the other day, with a different error code
19D7UMY. The reason turned out to be an error in the Connect app, which uses an erroneous route URL, so while we do not have any idea about the specific meaning of the error code either and it would be really helpful to provide more guidance for the user encountering it, the error has been within the handshake between the Connect host and our app and caused by a bug in our code, so maybe that’s worth checking for?
We have third one. It started to appear just recently, we have no idea why.
(Interestingly, if go “back” with the browser, use CTRL+F5 to force-reload the page, then click the same link that originally produced the error, it will succeed the second time.)
Note: I reported this as a bug to the Developer Service Desk (as DEVHELP-6040).
The “something went wrong” message might indicate the following problem: An update on our investigation to solve intermittent iframe loading problems in Connect apps - #18 by nick
There is also a bug reported: [ACJS-1156] - Ecosystem Jira
Short update others may find useful: Atlassian prepared a Connect mini-app to test with. The idea is that if this mini-app can reproduce the problem, then in our case, the root cause is on the Connect side, not in our actual app.
Result: the mini-app can reproduce it.