It is not unusual for your network security guys to force regular password changes. What is unusual is that once you changed your password you can no longer run NAV RTC. And receive the obscure message

Equally frustrating is that NAV Classic still works.

You will also see Event Log errors like this one on the NAV Server Tier

What's happening is your connection details have been cached (without you realising)

You can remove this cache entry by running the command

rundll32.exe keymgr.dll, KRShowKeyMgr

And then deleting the connection that represents the NAV Service Tier