iOS 10.3 seems to handle self signed certificates different then iOS 10.2. Even with the option NSAllowsArbitraryLoads enabled the request fails with the error:
The certificate for this server is invalid. You might be connecting to a server that is pretending to be “Domain name” which could put your confidential information at risk.
To fix the issue install the self signed certificate as usual. Afterwards go to
Settings -> About -> Certificate Trust Settings
and just enable the Full Trust for your root certificate.