CO.CC replied

I send a help file to CO.CC site, ended up that they has some technical issue.
They replied me in less than 5 hours, seems that this free service aren't as non-trustworthy as other free service now though, though errors might come out once in awhile.

Or maybe they thought that the address has been abandoned/inactive, hence they barred it.. Not a good idea, since my blog gets a minimum of 2 visitors per day.