Hey Andreas,
Most probably what's wrong is your joomla version - 2.5
Best would be 3.x, but I agree that it's not always that easy.
For starters - upgrade to joomla 2.5.28. Your joomla version has security holes that were already fixed.
On joomla 2.5 the mootools version that is provided breaks the google maps js. There is a bind function in the library that screws the map (on joomla 3.x the version of mootools is different and that bind function is not there). Anyway the way to go around this is to load the google maps api first, and after that mootools. We do this already in the latest release of Hotspot which is 5.1.10. And I no longer have a clue what we did in Hotspots 3.6.2 - this was like 2 years ago
Another possibility for the issue is - jquery conflict. Maybe you've have several version of jquery on the page and they can't work with each other. Anyway - without a link to the page I'm giving you ideas at random.
Regards,
Daniel