First, you should have had to install the forum sitemaps module after you installed GYM, should have been automatic. Did you deactivate it after you first installed the mod ?

Then, the error page you posted from your google webmaster's account is for your sitemap. This one for example -http://www.drakken.it/forum/rss/rss.xml.gz is marked as have problem, since it only contains three urls, I assume that the error is talking about urls having the same priorities, and than you lowered the default threshold for sitemaps (not very useful since you do not need a sitemap to display three urls).
In all cases, this has no relation to your forum indexing, it's only concerning sitemaps.

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I have deactivated it and activated again. My phpbb installation is phpbb-seo pack and GYM have been installed after phpbb.