Was struggling with using coordinates and not getting it to work. Using city,country as suggested in this thread did work well, but I would much prefer using coordinates.
After some digging I found that entering the coordinates formatted like below, it works:
Try using city and country code instead of lat/lon coordinates. Even though the Domoticz interface hints you can use coordinates, and the OWM allows you to do so, the implementation in Domoticz is broken.