… simply doesn’t work at all. Investigated to much time for this (some hours) without any results but this: it’s totally broken. What a pity. OTOH the phpBB3 devs do not support multiboard on VirtualHost in the first at all. So the Debian people tried hard but failed to find a convenient way to do this.
What a pity.
To have a multiboard running on the machine it is best to kick the update possibilities offered by apt (*sigh*) and go for the plain, manually install into the VirtualHost’s folders. Each with an unique phpBB3 installment. Yes, this works, but leaves the user with the nightmare in upgrading each board in separate, which is very, very cumbersome.
I hope the phpBB3 devs some day realize that multiboard and/or VirtualHost support is needed right out-of-the-box.