I found out there is a limit of the amount custom symbols when loading them. I don't know exactly what the maximum customs symbols can be loaded, but as far as I know it's about 500 symbols. My app was running about 3 days and than it crashes, as it has reached the maximum numbers of loaded custom symbols. I can't find anything about that on www and in MP help.
The only solution I can come up with is not to show all the pushpins that are too close together, especially when you look from a certain altitude. And when you zoom in, you see more pushpins, but only in the view (so seeing 500 pushpins in a view independently, will actually never occur ). I'd like to share an algorithme with you on how to do that, but I need one information, that's the relationship between the Altitude and the distance in pixels on the map. Something like a Altitude/pixel_on_the_map ratio. I hope somebody can share this wih us.