Whenever there is a problem with a NAV 3-tier setup, typically we check the same usual things to begin with (SPNs, Delegation, NAV Service account, etc).
Below is a link to a PowerShell script that collects some of this information for you. So rather than logging on to a customer’s NAV Server, then Domain Controller, then looking around in Services, ADSIEdit, Users and Computers, etc, just run the script and it collects most of the usual information in txt files.