Maybe, but not sure. You can compile an application against GTK 2.8 and run it with GTK 2.10. So, perhaps the configure script detects first the GTK 2.8 development files and use them. But when running Geany, the runtime libraries from GTK 2.10 are found first.
I guess you were right, i removed 2.10 and now geany runs :-))
Thanks for all your support.
Regards Sascha Heid