offline writing error - XML-RPC Response Parsing Failed

Dear Expert,

suddenly we can not send our post via offline writer (e.g. windows live or the MarsEdit). A few days ago, everything still run OK. IK'm not very sure what is going on. I have installed a number of WPMU Plugin, and also updated the BuddyPress to the latest version. The error message was:

the server reported an error: XML-RPC Response Parsing Failed: The operation couldn’t be completed. Extra content at the end of the document

What could be the mistake?

Appreciate your help.