I've noticed that some Anti-Virus products don't appear to be updating, but after rebooting or waiting approximately 5-10 minutes the updates come into effect. My guess is that these programs are simply not reporting that an update is in progress when in fact it is.
I'm not sure about Avira in particular, but I've seen this occur with F-Prot Anti-Virus, McAfee Anti-Virus, Norton/Symantec Anti-Virus (many times), Clam Anti-Virus, and AVG Anti-Virus.
Other possibilities to consider:
Avira's update server on the internet is temporarily down. Hopefully they'll resolve the problem soon (and update their software to report when this error occurs in the future).
Locally installed SpyWare is interfering with attempts by Avira Anti-Virus to communicate. Some SpyWare (most notably "Windows Anti-Virus 2011" and other very similar variations on that name {usually only the year changes}) will go so far as terminating a variety of Anti-Virus products and some will even uninstall them.
There really are no updates. Do you know for certain that an update is available? Virus authors don't adhere to consistent releases of new viruses, and it is possible that no new viruses are discovered for a while which could explain why you're not seeing any updates.