Sorry for the late response,
Hmmm, if there's nothing in the logs it seems like somehow it's not reaching the Mattermost server. If you DEBUG level logs showing then when the server receives a incoming webhook POST you should see something along the lines of:
[2016/03/09 12:03:35 EST] [DEBG] /hooks/kjh124hjkjg12h4ghgkg12k4jgkjg
Using SSL and a self-signed cert shouldn't matter for incoming webhooks.
Can you give me a censored version of the webhook URL that you're giving the integration?