My sister can't right-click anything

I decided to start a survival on my Aternos server with my sister, but after a minute or two of playing, she can't right-click anything and she has to restart Minecraft to fix it, only to have it happen again. We're both using 1.10.2, but I'm using OptiFine...that wouldn't make a difference, would it?
I also know this is serverside, as we usually play a minigame together where you have to right-click chests to get items, and she's never had any problems on there before.

Any ideas?

Quote from FaZeDarkStar

Have you checked her controls to make sure there aren't conflicting binds

I don't think she's ever changed her controls, and besides, she can right-click things for a minute or two, but after that, she has to reboot Minecraft.

Nope, that didn't work...however, she was able to place a sapling. Any other ideas?