I tried all these things.
- Create new mail profile
- Setting the login/email address to email@example.com
- Set DNS servers to 188.8.131.52 instead of internal DNS
- Create and connect to VPN without split tunnel
In the end, I found that the fix was to 1) create a new Windows profile and 2) disable the crappy Trend Micro antivirus. Of note, I recreated the profile at first wihtout disabling AV and I had trouble configuring email - so perhaps disabling the AV was necessary.