Hey Sebastian,
There is a bug in 5.0.29 where the "deny" rule wins the "allow" rule. It's not really a bug per-se but this was actually a feature requested by a customer. Having said that, we might have mislooked where the admin shares the same group as the "Registered".
To fix this, download the attached file and upload it into /administrator/components/com_easyblog/includes/acl/