QSync login failure on one machine

[EDIflyer]

I'm running the latest version of QNAP firmware (451+ NAS) and the latest Qsync software.

However I've noticed that my desktop PC is no longer syncing with my NAS (it was fine for the past few months). By comparison my laptop is working fine. Both machines are running Windows 10. The NAS is located off-site, therefore being accessed via myqnapcloud. I can access the web interface for the NAS fine from either device.

I tried uninstalling and reinstalling the Qsync software but just receive an error re 'login failure', despite using the same user ID/pwd as works for the web interface.

Any suggestions? I can't see how it is a NAS issue given the laptop is still working fine, so it seems like it is more likely something on the PC but I haven't installed any other software recently that I can recall, other services (incl Dropbox, Onedrive, etc.) all work fine and as I say I can access the NAS from the PC.

Thanks in advance.