Questionmark in black diamond instead of umlauts/special characters

I’m getting the replacement character (questionmark in black diamond: � )
for umlauts and special characters. This happens in the back- and front end. It appears at contents of this plugin only.
Cannot share site url at the moment, unfortunately.
Would be really happy for any advice anyways!
If I should check settings of charset to utf-8, I’d be happy for recommendations where I have to look for.

https://wordpress.org/plugins/gd-bbpress-attachments/