Cannot save report to server after adding a 2nd dataset

Hi.

I built a report using Report Builder 3.0 and I saved it to my server. The report works ok!

After adding a 2nd dataset (from the same datasource), it finds the fields and gives me no error, but when I try to save the report it gives me the following error:

Unexpected end of file has occurred. The following elements are not closed: Message, MoreInformation, detail, soap:Fault, soap:Body, soap:Envelope. Line 1, position 63684.
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Response is not well-formed XML.
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The attempt to connect to the report server failed.  Check your connection information and that the report server is a compatible version.

When I try to save it to my local computer, it saves it without problems.

Any suggestions?

Thank you!