Hi Markus,
Subsim forum is built on vBulletin, not PhPBB, look at the bottom of the page. An old version of vBulletin, but I cannot upgrade without uprooting a lot of features, like downloads. I'm ok with that, it's function that matters to me and this forum software is built like a tank.
If you want to start a forum with your friends, I suggest
XenForo. It's very active, well supported, robust and has a nicer UI than vB. You can get a starter package with hosting for $60 a month.
Or you can buy a license for $160 but you have provide the hosting. Shared hosting can be pretty cheap, less than $10 a month, but that's for sites with small traffic. If it is mainly you and a few friends, that's plenty. With shared hosting, your website lives alongside others, which are neighboring users utilizing the same server. With dedicated hosting plans, your website has a server all to itself. Because of the amount of bandwidth SUBSIM uses on forum posts and mod downloads, we have a mid-tier dedicated server.
Maybe you can start a forum on Reddit? I use it occasionally, but I really don't know what the rules are for starting a sub reddit (seems like anything goes).
Or start your own Discord server, super easy.
Neal