How to determine reason for blocked sites?

Hi,

The Sensei report showed me some blocks for a side, which my Android phone tried to connect to (sheldon.onebackend.de). No idea which app is responsible for the query.
I also havn't found any information about that URL, so I want to know why Sensei detect it as "Potentially Dangerous Sites". How can I check why Sensei reports it as bad?

Thank you.
Jas