Thank you for your replies. I registered the domain and set up a forum using phpbb back in 2014. (domain is still registered)
I kept getting notifications about newer versions of phpbb, so finally decided to bite the bullet and update from 3.1.1 to 3.2.7.
I tried using the suggested Installatron program using an FTP, but my router doesn't allow this without port forwarding. I have done this years ago playing on Xbox360, but can't remember exactly how to do it.
So instead, I followed the instructions listed on the admin panel, backing up first, and then loading all the files as indicated (install/ and vendor/ folders, retaining the file structure as indicated).
Curiously, there are two different sets of instructions to update to a newer version (I have tried both, without success)
I get stuck/don't understand if I'm doing correctly on the step:
instruction set #1: "point your browser to the install directory, for example http://www.example.com/phpbb3/install/"
instruction set #2: "Browse to /install/app.php/update"
As a last resort, I will install the 3.2.7 Full Package, instead of the update.
Any further suggestions are welcome and appreciated.