Copyscape has been down for around 24 hours now and there has been no official confirmation of any internal error so far.
The service relies on heavy use of the Google API so they are in a pretty precarious position if Google decided to break some of their functionality.
What is Copyscape’s relationship with Google?
Copyscape is unaffiliated with Google, although we are in regular contact with them. Copyscape uses Google’s Web APIs to perform its searches. Google has generously provided us with a high-capacity Web APIs key for use with the Google Alert and Copyscape sites.