My b2evolution Version: Not Entered

Hey Guys,
just installed the latest Version of this great software - and everything worked really neatly, but there's several spots on my Blog where this message appears:

"Fatal error: Allowed memory size of 16777216 bytes exhausted (tried to allocate 7680 bytes) in /srv/www/web91/html/webspacexxl-de/blog/inc/sessions/model/_hitlist.class.php on line 54"

Sometimes it's at the bottom of a blog, sometimes in the administrator-control-panel.. everything is set on default, there is still the three default-blogs there, I just made one entry in "Blog A", and changed the admin password.

I've read about this problem in the Forums here, but nobody hat 16M memory (like I seem to have), so I thought this is a little weird..
And I have safe-mode OFF on my server, but I was NOT able to increase the memory with "ini_set( 'memory_limit', '20M' );". Also, I thought 16M was enough?

Thanks for any help!
Flo.