Since this bug is pretty gnarly, we wanted to put out a quick update about.
We just released a hotfix - so restart Steam client to be sure to get it - that fixes a bug that was introduced in Update #113 last night.
If you have any issues with missions freezing on the first turn or when reinforcements are arriving, please be sure to restart Steam client and get this latest update.
All should be fixed now and very sorry for the bug!