Is it happening every time you use ctrl+] / ctrl+[ or only when you're viewing replays from a specific folder? Does it start new game if you use these hotkeys right after game client start without going to free play? Do you use any third party software that forces custom keyboard mapping?
Please also make sure you're on Toribash 5.41, if you're on Steam version then also try verifying game cache. If it still doesn't work properly, please replicate the bug in your Toribash client, close it and attach the generated stderr.txt file here.