I replied you in PM, rule of the thumb - disable mod_security, it wont help much and is unofficial mod anyway.
If you can't get your hands on apache config try adding .htaccess in gallery root with:
SecFilterEngine Off
Also there is like half zillion options so if anybody want to spend some quality time with configs, try to switch them on and off, that might come in handy.
For now I would say that mod_security is half- or not-compatible with any Flash uploader.
About error 406:
http://urbangiraffe.com/2005/08/20/myst ... 406-error/