Before doing anything else, I recommend you power down all computers and devices (network printers, cable or dsl modems, router, switches etc.), then power up cable or dsl modems first, wait several minutes, then power up router, then power up main computer first, navigate your browser to the router, and then try saving new password or other settings. If there is still a problem after doing that, try a different browser (like if you were using IE, try FireFox.
If your firmware version is 1.21, depending on what you are going to do with the router, I urge you to consider staying at that version. All the firmware updates since 1.21 have caused a lot of problems for many users. It has caused me so much trouble that I just went and bought a Linksys router to replace one of my D-Link DIR 655 router. I am tired of fighting the D-Link router for nearly a year now. Most of my problems have been regarding VOIP.