Hello,
after upgrade from 1.12.15 to 1.12.17 all IP's in manual entries eth interfaces stops working. You have to change users assigned to this interfaces to another one, unassigned resellers, remove from IP management, remove manual entries from /etc/network/interfaces (Debian) and add it again via IP management, again assign Reseller(s) and User(s).
I spend few hours on this issue, I hope that this mini "how to" helps someone. I don't track this bug, becasue I think this is best solution, you do not need to make changes in the I-MSCP software engine.
Best regards