Hi,
the problem isn't within the template. Chosing another template just didn't display any modules, so after some tinkering around, I found out there's a PHP Critical Error whenever a module is called on the JomSocial sites which uses the s2framework.
Specifically, the error message is:
"PHP Fatal error: Cannot redeclare Aws\constantly() (previously declared in phar://aws-3.138.7.phar/Aws/functions.php:20)"
I saw there are some posts pertaining to an incompatibility of JReviews and JomSocial. Both worked fine until a little while ago, so I do not know whether it's a change in the code of JomSocial or JReviews. Will contact the JReviews developer as well, but would ask the JomSocial team to look into this just as well.
Solution for the time being: Disable any JReview modules on the JomSocial pages.
Thanks for looking into this!