Server: Using /op from the console doesn't work when server is in offline-mode

Because my player data was deleted, I somehow lost my op status in my own server. I tried to /op myself using the console, and it stated that it worked. When I got back into the game, it wouldn't let me run op commands. I tried both "/op <username>" and "op <username>". Once I tried substituting my UUID. The UUID gave another error that I don't remember. After a while of fix attempts, it won't even let me back into the server. It gives Internal Exception: io.netty.handler.timeout.ReadTimeoutException.