The last 1.7.10 release is quite old, so before we delve into debugging please give this current development version a try: https://minio.johni0702.de/replaymod/replaymod-1.7.10-2.1.2-11-g65e3d45.jar
Which modpack is that? Unless you can narrow it down to just a conflict between the RM and one or two other mods, it'll probably be best if I can reproduce it.
To not get your hopes too high, it'll be better if I warn you right now: We do not put much priority on the 1.7.10 version. So, whether this gets fixed may very well depend on whether on of us it sufficiently bored to do it. That may happen tomorrow, in a week, just in two months or not at all. It'll also depend how much work the fix requires, if the issue is obvious and the fix is trivial I'll probably have it fixed within a day.