Hi,
It does not appear to be related to FCKeditor. The error looks like it is failing to parse a datetime from a string, like for the start date of the post. Most likely it is a localization issue.
Are you focring a particular culture in your config settings?
Do you have your browser language preference set to something other than English? If so what language?
I suspect that whatever the string in the DatePicker for the start date it is not able to convert it to a datetime object.
Hope it helps,
Joe