misnina go into your admin panel, open Flarum PWA settings and click on "Regenerate VAPID keys" & clear your cache, it should fix your issue
Wulfheart by the way, maybe have a look at this thread, it might help you if you decide to self-host Flarum - the migration software seems to support phpBB directly, so a CSV conversion might not be needed. Let me know if you need help, or reply to that discussion