Flag count wrong with anon sessions

Hi everybody, I have the following situation with Flag 6.x-2.0-beta5 and Session API 6.x-1.4:

- I created a flag called "like"
- permission to flag and unflag is granted to anon and authenticated users
- un-/flagging is done by ajax
- the flag is configured to not show on node teasers and node pages
- output is done with print flag_create_link('like', $node->nid); in node template files
- Drupal "normal" cache is activated

We encounter several effects when flagging / unflagging, independent of which browser brand we use:

I. First click does not change the displayed flag count (anon & auth)
Often after you hit a flag you get the visual feedback (I disabled message display) that the system recognized the click (the flag disappears and instantly reappears) but it displays the same count as before. Only when I click a second time the action seems to trigger and the flag count goes one point up / down.

II. Flag gets it all wron on subsequent click (anon only)
If I encountered the problem described in I. and do a third click, the count often goes down two points.

Example (all done without in-between page loads):
- node has a "like" flag count of 1
- anon user hits the flag once - flag count disappears and immediately reappears but stays at 1
- anon user hits the flag a second time - flag count goes up to 2
- anon user hits the flag a third time - flag count goes down to 0!
- anon user hits flag a fourth time - flag count goes up to 1
- subsequent flag hits only switch between 1 and 0

I thought it may have to do with the fact that I test the site as anon user and with two different accounts and flag and / or session api gets confused by switching accounts or being anonymous with the same session api session (as I understand it and verified with the firebug firecookie plugin, the session api cookie is the same no matter what). But even that would not explain why nothing seems to change on first click and why I'm able to change the flags value by two with one click.

You may check the behaviour by yourself on the following site: http://www.digital-room.de/