When De-opping or opping players and writing their names, if you write their names non-cap sensitive (ex. AmokaHD (true name), write amokahd) You receive a fatal error message in Web Console.
This can only be resolved by deleting and then re-adding the jar.
Steps to reproduce:
Host a server
Try to op a player or deop a player writing their name correctly but not case sensative.
Please attach the crash report (path -> See your screenshot)