Error adding another Google account Device: T-Mobile G2 (UK) / HTC Hero OS: 2.1

I have a primary gmail account, a second Gmail account, and a Google Apps account. I'd now like to add another Google Apps account.

However, whenever I try, I get a "you don't have a network connection / This could be a temporary problem or your phone may not be provisioned for data services. lease try again when connected to a mobile network, or connect to a Wi-Fi network. If the problem continues, call Customer Care" message. However I've tried this from 3 cities, across a few weeks, from where I'm always able to access internet and Google accounts.

I presume there's no limit of 3, so what else can I try?

Edit - 3rd Jan - I just tried removing an existing (working) gmail account. I can't add the new account, or put back the removed account - new error is "Can't establish a reliable data connection to the server./This could be a temporary problem or your phone may not be provisioned for data services. If it continues, call Customer Care.". This is on WiFi and 3G - whilst the process was trying I received (pull) email! There's something really wrong.
I have 4 Google accounts, so no there's not a limit of 3.  I'm guessing trying two Apps accounts is the problem.  Try adding another standard (gmail) Google account and see if that works.

Edit: I'd try a factory reset at this point.