I was also recieving the "Exchange server requires secure connection or account is disabled" error. I had exported the certificate chain of our private CAs and imported them to the device with no change, either over WiFi or GPRS.

After finding the following bug report, I fixed it after seeing that what Labra had suggested was the case. (comment 17)

https://bugs.maemo.org/show_bug.cgi?id=6467#c17

The user-imported certificates are being imported 'into /home/user/.maemosec-certs/ssl-ca', and MfE only seems to look at certificates in '/home/user/.activesync/certs'. Copying the imported private CA cert files allowed me to sync.