I thought you said you were on php 7.2 now
As posted ubb is not ready for php7.2 , even with the pending release of ubb 7.7 in fact I am not aware of any forum package that will accept php7.2 at this time.
There are just a very,very few that even accept php7.1 that I know of currently.
However your version of Ubuntu 18.04 will accept downgrading to php7.1 from what I read.
Still not low enough for ubb's current release..
As I suggested, I would work on making your classic version work again for now.
Check your path to perl and disable the accelerator if it is turned on.
Then it will just use perl(cgi scripts)
And or discuss with your host about how to down grade the server to get to php 5.6 for now to upgrade ubb classic to Ubb threads..
We can't help much more that that without a url or access to the site.
And I have never had a lot of success with using the re director script after upgrading