Non-Operator Players can't use /whisper, /msg, or /tell. "insufficient permissions for selector expansion"

Players on my server are unable to run the /whisper or /tell command suddenly.

To replicate:
Be a player with no OP permissions, type “/whisper” or “/tell”, try to send a message with the selected target.

Expected result: Message sends as whisper to one player

Actual result: Player is met with "insufficient permissions for selector expansion"

We’ve not changed any settings on the server to disallow whispers, they were working fine before they reported the issue today.