[Plugin: GRA4 Social Network] rewrite_rules_array. 404 with POST

Hi all.

In my plugin I’m doing something like

function gra4_set_seo_friendly_rules($args)
{
	$new = array();
	$new['(gra4)(/[-/0-9a-zA-Z]+)?/(.*)$'] = 'index.php?pagename=$matches[1]';
	return $new + $args;
}
add_filter("rewrite_rules_array", "gra4_set_seo_friendly_rules");

I also do $wp_rewrite->flush_rules(); on init.
I guess it should cover all URLs ‘longer’ than my plugin page (the plugin inserts content into a page),so anything like:
http://myserver.com/wp/gra4/whatever/i/put/here
pretty much is reflected to:
http://myserver.com/wp/gra4
with no 404 pages.
That’s pretty much how it works. Well, almost =)
On some URLs it still fires 404. What really surprises me, it happens only when POST data is present – the same page with GET only works ok. It also happens not to all URLs (so far I discovered only two malfunctions, allthough both with plenty of POST data), majority POST pages work just fine.
I commented all my code out… well, it does not matter, I guess when I see This is somewhat embarrassing, isn’t it? it does not even hit my code yet, right?

Any ides? I kinda lost, don’t even know where to start digging…

Thank you.