After going through the dev Exchange server logs I found , that the failing connections did not have a sequence cookie on the Exchange servers. With HTX on this happens and so far only with Outlook 2010 using MAPI. Outlook 2013+ work fine, turning HTX off will show the cookie on Exchange logs with Outlook 2010.
In the image below, the first lines are connecting through haproxy, after the red line connecting directly to Exchange.
