being the original poster I would like to give a feedback on how things went on in our case in the meantime. Jesse was suggesting to have look into the apache log and it turned out that apache did have a hell of a lot of problems with every request designated to RT (shame on our sys admin that he did not check this in the first place). This apache instance also is the “gateway” for a content management system based on php I believe called typo 3. So we blamed the whole thing on some configurational interference.
Short of time for further investigation our sys admin simply setup a separate apache instance on another virtual machine solely dedicated to RT. This one now works without hassle. I still asked for the error log of the original instance but did not get it yet. If there is any interest in this forum to still track this down I will be happy to provide it.