hello Simon Blomqvist ,

Sorry for late reply to this,
I have checked in your site, it seems like you hitting this Fatal error when click the button :

<br />

<b>Fatal error</b>:  Class 'SocialEventAppGuestsHookNotificationComments' not found in <b>/www/webvol3/2e/hg8fkmmyylcbcbv/engines.nu/public_html/administrator/components/com_easysocial/includes/apps/dependencies.php</b> on line <b>617</b><br />

I have applied the fix in this file JoomlaFolder\media\com_easysocial\apps\event\guests\hooks\notification.comments.php, it should work fine now.