@Nick-B, I did some further testing (updated the original ticket):
I decided to try 8.2.0 and it works fine.
However, 8.2.1 also works fine, so I don't think it's AdminerEvo.
Tried 8.3.2 and it also works fine.
Works in 9.0.2 and 9.0.3
But it breaks in 9.0.4
It seems this has also been reported in Support #43914 Database connection over a month ago.
Since it breaks in 9.0.4, do you think someone could take a look? I have attached relevant logs to the original ticket.