I don't use Hidden Forums plugin. I also tried disabling all the plugins - still the same navagation issue.
Then I disovered that if I use "none" permalink structure (/forums.php?id=1) everything works fine. If I use either "numeric" or "name based" (my preference) the problem is back.
Really noone faces such an issue?
my .htaccess looks like that now:
# BEGIN bbPress
Options -MultiViews
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteRule ^page/([0-9]+)/?$ /index.php?page= [L,QSA]
RewriteRule ^forum/([^/]+)/page/([0-9]+)/?$ /forum.php?id=&page= [L,QSA]
RewriteRule ^forum/([^/]+)/?$ /forum.php?id= [L,QSA]
RewriteRule ^forum/?$ / [R=302,L,QSA]
RewriteRule ^topic/([^/]+)/page/([0-9]+)/?$ /topic.php?id=&page= [L,QSA]
RewriteRule ^topic/([^/]+)/?$ /topic.php?id= [L,QSA]
RewriteRule ^topic/?$ / [R=302,L,QSA]
RewriteRule ^tags/([^/]+)/page/([0-9]+)/?$ /tags.php?tag=&page= [L,QSA]
RewriteRule ^tags/([^/]+)/?$ /tags.php?tag= [L,QSA]
RewriteRule ^tags/?$ /tags.php [L,QSA]
RewriteRule ^profile/([^/]+)/page/([0-9]+)/?$ /profile.php?id=&page= [L,QSA]
RewriteRule ^profile/([^/]+)/([^/]+)/?$ /profile.php?id=&tab= [L,QSA]
RewriteRule ^profile/([^/]+)/([^/]+)/page/([0-9]+)/?$ /profile.php?id=&tab=&page= [L,QSA]
RewriteRule ^profile/([^/]+)/?$ /profile.php?id= [L,QSA]
RewriteRule ^profile/?$ /profile.php [L,QSA]
RewriteRule ^view/([^/]+)/page/([0-9]+)/?$ /view.php?view=&page= [L,QSA]
RewriteRule ^view/([^/]+)/?$ /view.php?view= [L,QSA]
RewriteRule ^rss/?$ /rss.php [L,QSA]
RewriteRule ^rss/topics/?$ /rss.php?topics=1 [L,QSA]
RewriteRule ^rss/forum/([^/]+)/?$ /rss.php?forum= [L,QSA]
RewriteRule ^rss/forum/([^/]+)/topics/?$ /rss.php?forum=&topics=1 [L,QSA]
RewriteRule ^rss/topic/([^/]+)/?$ /rss.php?topic= [L,QSA]
RewriteRule ^rss/tags/([^/]+)/?$ /rss.php?tag= [L,QSA]
RewriteRule ^rss/tags/([^/]+)/topics/?$ /rss.php?tag=&topics=1 [L,QSA]
RewriteRule ^rss/profile/([^/]+)/?$ /rss.php?profile= [L,QSA]
RewriteRule ^rss/view/([^/]+)/?$ /rss.php?view= [L,QSA]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^.*$ /index.php [L]
</IfModule>
# END bbPress