Hello,
Thanks very much for your answer.
The problem has been partially resolved because the first computer is now syncing properly again. I finally found the cause of the problem, which seems to have been a conflict with my VPN (Surfshark). After uninstalling and reinstalling the programme, everything back from December was synced.
However the second (main) computer is still running round in circles saying "syncing remaining 1 file". I suspect that your third point (You are trying to synchronise an unsupported file type or a file with an unsupported file name.) is the cause of the problem because there is one file that isn't syncing. I will follow your advice and contact the hotline but in the meantime, all recent files apart from that one have been syncing normally.
Many thanks again for your help.