Thanks to Tom, I looked more carefully to the plugins.
And I can confirm that the problem is NOT RELATED to mod_rewrite, but to a plugin conflict. (sorry that I posted such a suspicion, but I read somewhere a very similar problem that was explained like that.)
Deactivating plugin by plugin, I found that one you made this behaviour. Still no clue why, but at least I have now a workaround: deactivating this plugin.
Thanks Tom and all the others for your help!