That message is usually due to the browser having an inconsistent page in it's cache. If you clear the browser cache it should work.
Also, where you can, you should disable offline caching and and any browser/system addons (things like password managers, adblockers) for the router web pages.
If you are using firefox, to disable the caching....
Tools-> Options -> Advance -> Network -> "The following websites are allowed to store data for offline use" dialog. Remove the entry for the router.