luceos had now a full day off and i've managed to make things run again, so Flarum works now for me.
The problem was in my case actually a lapidary one and it had something to do for the semi-descriptive titles for forms.
I've added my (email) control panel username into the field which is literally a username and not an email address 🙂 .
So the descriptions for the input forms weren't clear enough for ->ME<- in the SMPT settings.
If i may make a suggestion, please make use of ::placeholder texts for forms like for example in the SMPT setting for the Username form -> (Placeholder: Email etc).