Then the only initial criteria at the point where you hit the ‘Open’ button has been met.
Meaning once that request hits the server, server says no. Which means that a process (AI/Human) has decided that you can’t have a JA. And that’s not personal but your account has been labeled, along with many others it seems, for some reason, into the server-says-no group. And it’s that unknown reason which makes no sense.
I suspect - as it has been said before - that Monzo is really throttling the number of JA’s that can be opened. Possibly down to none at some stage. And as I don’t really see Monzo limping along with a limited number of users with a disjointed JA the way it is, the only outcome of that would be the ultimate demise of the 
Joint Account or hopefully a rebuild & relaunch of JAv2