If you are using google ping, please deactivate it, yahoo and msn recently started to refuse sitemaps pings with the official method.
Then, what happen when you load you sitemaps your self ? and if you clear cache before you do ? In no error show up, then chances are great that it has nothing to deal with GYM, something like a server reboot or overload (not necessarily due to gym).
Then about the premod and indexing issue, please use the appropriate forums.
Though, it seems that you did not activated the zero duplicate and did install your robots.txt (thus did not pay enough attention to all the warning about it in the install file) before you could redirect anything, so just delete you robots.txt (at least anything dealing with viewtopic, index and viewforum) and activate the zero dupe with the post redirection option at least set to "guests".
Though, I get 463 index page here : http://www.google.com/search?q=site:www.talkberlin.de
and your forum does not have this many topics, so with no duplicate you do not seem this far from full crawl already. But it's still worth it to start using the zero dupe and drop the robots.txt for couple months.