In March 2009, I set up a web address using DynDNS to point at my router, and configured the DDNS settings on the Vigor to update it when necessary. Last week I noticed that the web address was not working any more, though I have regularly logged into DynDNS.com to keep my "free" account current. The last DDNS log entries on my router, produced when I tried the "Force Update" last night showed:

23:39:40.5 >>>>> DDNS is updating. <<<<<
23:39:40.5 A= xxxx@WallaceHome.me.uk, H= wallacehome, U= 1
23:39:40.7 Connecting to the DDNS server (0xcc0df870)...
23:39:40.0 Return Code= badauth
23:39:40.0 Account updating error.

It looks as if it is failing to log in. My password on DynDNS has not changed, but to be sure, I reset it on my DrayTek router anyway, and tried again (the last few Force Updates were after doing that). The web address I have set up on DynDNS is wallacehome.webhop.net, and DynDNS shows it still pointing at 82.44.74.3 with last update on March 23rd 2009 (my WAN address is now 94.172.30.30). Has anyone on the forum any clue why DynDNS might reject my router's login?

Had a similar experience a couple of weeks ago, free dyndns account on my 2820n had expired.
Checking on the router I found that the dynamic dns update interval had changed to a gobbledegook enormous number. The only thing done since initially setting up was upgrading router firmware, I can only assume the number corrupted at that point.
I will certainly check it in future after f/w upgrades.
Regards

FWIW, if you use DtDNS instead of DynDNS you do not keep having to log into your account to keep it current. It is very useful if you are on VM cable service where your IP rarely ever changes.

Thank you both Telagente and NJH for your helpful suggestions. I cannot find a reference to a facility for setting the dynamic DNS update interval in the user manual for my 2910VG, but from the log, there are about 5 earlier entries (unfortunately only time stamped, not date stamped), plus one that happened just now when I opened, then saved my DynDNS setting, all still failing to log on. What is worse, my ISP (VM, I'm afraid), having given me an unchanged IP for at least most of this year, probably longer, has changed my IP yet again since I first looked at this problem 2 weeks ago. My guess is that the router only attempts an update when the WAN IP changes, but if that is so, I must have had at least 5 IP changes in the last 2 weeks (or perhaps it persists at intervals after an IP change until it succeeds?). Anyway, I have just set up an account on DtDNS following NJH's suggestion, and that worked immediately.