Hmm, I tried to figure out what the differences are between my E71 syncing with my Scalix Groupware Server and the way the N900 tries to.
What I noticed right away is that it seems that the first http-request is somewhat different as I can tell from my server's logs:

(All IMEIs substituted for illustrational purposes)

E71:
2010-01-19 14:05:17,974 INFO [WirelessServlet.doPost:103] SERVLET - deviceId: IMEI12345, user: XX - HTTP post Request received: /wirelessUser=XX&DeviceId=IMEI12345&DeviceType=IMEI12345&Cmd=Sy nc
2010-01-19 14:05:54,099 INFO [SyncCommand.doSync:194] COMMAND - deviceId: IMEI12345, user: XX - executing content sync on folder: Kalender

N900:
2010-01-19 14:19:05,822 INFO [WirelessServlet.doPost:103] SERVLET - deviceId: 54321 - HTTP post Request received: /wirelessCmd=Sync&DeviceId=54321&DeviceType=SmartPh one
2010-01-19 14:19:05,825 INFO [SyncCommand.doSync:194] COMMAND - deviceId: 54321 - executing content sync on folder: Kalender

Where's the variable telling the server what user account the N900 tries to sync with? Is it in later EAS versions that the information on DeviceID vs. user account is stored on the server side?

--edit--

BTW that's the way my colleagues iPhone syncs (flawlessly):
iPhone:
2010-01-19 14:38:00,813 INFO [WirelessServlet.doPost:103] SERVLET - deviceId: Appl12345NR, user: YY - HTTP post Request received: /wirelessUser=YY&DeviceId=Appl12345NR&DeviceType=iPhone&Cmd=Syn c
2010-01-19 14:38:00,880 INFO [SyncCommand.doSync:194] COMMAND - deviceId: Appl12345NR, user: YY - executing content sync on folder: Calendar

E71 & iPhone pretty much show the same logs. The N900 is missing the user-field...