And the answer is......
From the error message, that board is running phpBB 2.0.x, which is no longer supported. The board administrator will need to update to 3.0.x or 3.1.x.
Instructions to convert from 2.0.x to 3.1.x are here:
https://www.phpbb.com/support/documents ... =3#convert
Scroll down to number 6
It starts with:
Before converting, we heavily recommend you do a full backup of your database and files
! If you are unsure how to achieve this, please ask your hosting provider for advice
. You basically need to follow the instructions given for New installations. Please do not overwrite any old files - install phpBB3 at a different location.
So I was probably right about cleaning up the board (i.e. deleting user accounts and their posts/signatures
for anyone who hasn't been here in 7 years or whatever.)
That will make the database backup quicker & mean less time offline during the upgrade.