HiDrive (STRATO) responded by saying they didn’t know why I couldn’t link Odrive and Hidrive and to ask you what the error code meant (I did mention you asked me to ask them). So that response wasn’t terribly helpful. I just tried again on the off chance and for some reason both my HiDrive accounts now linked to Odrive without any problems. So the problem appears to be solved, although I have absolutely no idea how. Thanks for your help though.