I absolutely don't think your problem has anything to do with NextPVR. Perhaps try the 32bit version of Kodi, maybe your problem (which I don't see) is related to 64 bit drivers.
The backend is pretty much physically inaccessible, in the roofspace of my house. I manage it using Teamviewer. However, it does have Kodi 17.6 on it and I could perhaps run some remote tests that would remove the network and the client PC from the scenario. As a last resort, I could also retrieve it, extend the terrestrial aerial connection to some more accessible location and attach a screen etc to check things without Teamviewer being involved also.
The logs show the network is ok, there is a regular call to the backend that is going through until Kodi closes. In addition to debug logging maybe enable PVR and EPG component logging in Kodi.