OnePlus One (CM 13.1.2) persisting boot loop
After contacting OnePlus they set up a remote session.
During this session they also flashed the phone using their own files.
This, however, did not solve the problem.

The phone was still under waranty so they arranged it to be send for repair.
They found the motherboard to be faulty, which has been replaced and the phone works perfectly fine again.