It's been pretty messed ever since season 3 came out.
I just thought I'd help you guys out. I found a fix by RyuuuGaming on YouTube but I think it's in Spanish (not sure). Anyway, I made a guide out of it, this time in English:
This "Fix: Lolreplay Not Working After Patch 3.5 [Solved]" guide circumvents the Season 3 bug. (It addresses the "Unable to locate the League of Legends.exe" bug but it might help you guys nonetheless.)