it seems that for phpBB 3.2 you need to modify the UrlToolkit, instead of Match [^?]*(\?.*)? Rewrite /app.php$1
it must be Match ^(.*)$ Rewrite /app.php/$1
(taken and adapted from the nginx config example at phpBB)
Latest phpBB seems to have replaced the '?' in some Urls, especially in the installer, with '/'.
The UrlToolkit which worked for me to install phpBB 3.2.2 is:
ToolkitID = phpBB
RequestURI exists Return
# Match [^?]*(\?.*)? Rewrite /app.php$1
Match ^(.*)$ Rewrite /app.php/$1
Hugo, happy holidays to you!