birdie - Regarding your errors, it's because you installed the modification on the same server with multiple vB forums, I assume you used either XCache or Memcache? No doubt the cached items conflicted. This will be rectified in 1.0.3 where you're given the ability to insert custom data prefix's to ensure the cached content is kept unique to each individual vBulletin install.
ForYou - Unfortunately I've not been able to replicate this issue, if you could turn off cache phrases, then cache styles, then cache templates, and after each turn attempt to use quick reply again, to see if disabling any of the cache methods fixes the issue. If I'm right then it'll be phrases, since you're using a different language. That or something specific with your apache setup.
Andreas - Thanks for the kind words, and for the suggestions. I did consider eAccelerator but unless I'm mistaken vBulletin doesn't use it for a bug that occurs, if or not that will relate to vB Optimise I'm unaware of, but I'll look into to and if not integrate the controller.
As for everyone else, I'm happy to see the positive impact this is having on forums, I'll be releasing 1.0.3 very soon