Generally when we see this error, it's due to a conflict with the time that the server is reporting. We'll produce this message when the timestamp is in the future or WAY in the past causing weird things to happen. We definitely recommend checking to make sure that's accurate.
In one instance, there was an issue with WP Super Cache that was causing one particular person to see this error. However, I haven't been able to reproduce the same behavior on this end of things. You could try to disable a couple plugins and slowly turn them back on to see if it's one particular plugin (or combination of plugins) that's causing things to go haywire.
The last option is that the code has been edited. If that's the case, you could simply remove the plugin and re-add it through WordPress for a fresh crack at it.
Let us know if we can help with anything else!