Replying to Discussions - Value does not fall within the expected range

Hi All,

I have a new Community Site in a SharePoint 2013 environment. Occasionally both myself and other users are unable to reply to discussions in the default Discussion List provided by the site template. When clicking 'Reply' and typing in a message of any length (1 to many characters) a popup is presented with the following message:

"Value does not fall within the expected range."

This appears to happen to Discussions started by a particular user. Other users added to the site from the same AD Groups do not cause this issue.

Any help or guidance would be incredibly helpful. If there is further information I can provide please let me know, this is my first post here on TechNet so be gentle if I've not followed any standard procedure! :)