Hi  Check if bearing cones are spinning in the drum.

It has taken a while to come back to this page, and probably way out of date or memory of those who replied in 2021. But in late September, after spending many dollars on mechanics etc, I found the fault through a process of elimination.

I disconnected every thing, the ESC, the electric brakes, to the connection of the  offending wheel, to prove it was not mechanical. Went for a drive all good, reconnected electric brake, all good. I found it was the ESC, for some reason it was sending a  mild signal to that one wheel brake, which just allowed the brake to apply minimal force. I unplugged the connection under the van, cleaned all the connections, and hey presto problem solved..

I did contact AL-KO  for advise earlier in the piece which was a total waist of time, after 9 phone calls over 12 days, a women answers the phone, and says she is transferring you to their tech department, who, never answer the phone, leaving a message does nothing either.