I experienced this issue as well. I installed NextPVR as the administrator (for Windows XP) and following some setup everything worked well on the administrator account. However, when I logged in to an account which does not have administrator rights a problem arose. In short, NextPVR was unable to make entries in the NPVR.db ... despite the fact that the database and other supporting files were located in:
C:\Documents and Settings\All Users\Application Data\NPVR
directory. I attempted (as administrator) to set sharing options to this directory but the problem persisted. Only when I copied the contents of this directory to:
C:\Documents and Settings\All Users\Documents\Application Data\NPVR
was NextPVR make entries to the NPVR database. The key to the solution was putting the sub-directory in the All Users "Documents" sub-directory. In addition I had to change (using regedit) the registry entry (DataDirectory item) to point to the new location.