JoaquinBL I can't reproduce this problem on my end. Have you been able to watch replays from past patches (i.e., not the then-current patch) at some point before? If not, please either try the "Repair replay client" option from the "Replay settings" menu, or try deleting the "replay client" directory in "...\Documents\LoLReplay2" and try again.
If that doesn't help either, maybe the replay file is corrupted. In that case, please upload it using the "Upload" button, and drop me the link.