Some Update here:

the EAS Protocol setting didn't help with egroupware, since egroupware is synced with the symcML Protocol.

I also tried a go with funambol, which as of now syncs with the N900 successfully! I now have to figure out, if just funambol is enough for what I need, or if I use funambol as a link between the n900 and a groupware server. Some work ahead!