What's interesting is it still works for one ofg mu hotmail accounts, just not both. As far as I can tell they are using the same protocols.
When you connect a Hotmail account to WLM it defaults to DeltaSync (email, calendar, contacts) I believe. This was fine until MS started upgrading Outlook.com and removing DeltaSync support.
DeltaSync is only available in the old Outlook.com experience. If your account has been upgraded to the new Outlook.com experience then DeltaSync is gone and WLM no longer works.
You can log in to Outlook.com and check your accounts. The account that still works probably hasn't been upgraded yet (old Outlook.com experience). If you log in with the account that doesn't work in WLM you will probably see the new Outlook.com.