On my server my players can't pvp. I have ruled out all of the plugins I recently installed. I looked in the console and it doesn't say anything about this error. I would like to have pvp re-enabled but how would I find the cause of it if the console doesn't say anything?
Have you checked your server.properties? I'm pretty sure there's a setting in there which allows you to enable/disable pvp.
Do you have worldguard installed? There's a flag that allows you to disable pvp which might be causing the issue.