"Exchange server not responding" issue happens when you try to sync a calendar in your n900 with google that was previously synced with anything else. (other mobile, exchange, pc suite, etc) For example I imported my contacts (so not calendar) from my other mobile phone, and after tried to sync with google, and it did not work.

Resoution:
In sync settings set up a new calendar, and sync will work again.
You can also delete all items from calendar named "N900" and then try to retrieve the calendar items from google.

It worked for me, but somebody please confirm this is working.

Istvan