I thought I would post this little snippet of information because it caught me out again and after an hour I resolved the problem again.
I use dyndns for a number of hosts and I find the the Draytek routers stop sending updates. There is a 'badauth' error in the log.
The fix makes no sense but you have to change the password on the dyndns account and then change the setting on the router, it then updates perfectly. Of course you have to then change the password back to the original to ensure that any other routers, that you may not have immediate access to, carry on working. When you put the original password back into the 'failing' router it updates perfectly. No idea why and clearly this is a problem, but the fix works.

Does it not fix also if you just go to the settings and 'save' the existing ones too
without actually changing the p/w ?