I've been very busy so a new blog post a little later than I really wanted to.. But this should help people that get the terrible error for Azure AD Registered Windows 10 devices of 'The sync could not be initiated (0x82ac019e)'.
Nearly every post on the internet for this error relates to an unlicensed user. However that's not actually always the case, in this instance it was a firewall configuration issue.
The device was Azure AD Registered by simply connecting a Work or School account to the device, however upon doing so and trying to force a 'sync'. This error presented itself.
Trying to get to the Azure AD registration url https://wip.mam.manage.microsoft.com:444 gave the following error.
Similarly after installing the Telnet Client the Windows 10 device couldn't open a connection.
This became evidentally clear that this was a port issue, most likely firewall related. After opening port 444 the Windows 10 device could talk successfully to https://wip.mam.manage.microsoft.com:444
and Event ID 209 showed a succesfully registration
And under | Settings | Accounts and sign-in | Connected Accounts | Info | If I selected 'Sync', it would now synchronise succesfully.
And my device was succesfully Azure AD registered!
As there were already succesfully Azure AD joined devices it became clear that there is a difference in the way both operate. Azure AD joined devices talk over port 443 which is almost always open on the firewall for outbound traffic.
Azure AD registered devices talk on port 444. You will most likely find this port is blocked in enterprise environments, and if it is, you'll need to open it.