I'll give a shot at reversing the leads and see what happens.

uppo: It is a DC supply. I have a picture of the board if a visual look at it helps - I noticed this new board does not have a secondary winding (plug X19) and had to use a different plug to connect to D1; could this possibly be the cause of overvoltage?

https://www.dropbox.com/sc/ds2zubdmnz4nr4v/AADDVPseCpHnYxOonEN2YVlsa