Tags lead to blank page

Hi,

We are running gamgea.com on WP. A few weeks ago, we changed the theme and converted categories into tags. Since then, tags don’t work anymore. They display properly, but if you click one, it does not find any posts using this tag (just an empty page saying that there have not been found any entries).

I switched back to default theme, no change. mySQL tables were repaired, no change.

There are a few user over the net, who seem to suffer from a similar problem, but no one has a solution for this. Any help is appreciated!