The Organization Service URL is invalid

Hi, I am trying to install ADX on a Dynamics 365 online instance.  When I type the Dynamics Organization URL, copy - paste from the Developer Resources, the moment I tab out of the field I get an error

The Organization Service URL is invalid. Metadata contains a reference that cannot be resolved: 'ahdemositex.api.crm.dynamics.com/.../Organization.svc'. There is an error in XML document (1, 1298). The maximum nametable character count quota (16384) has been exceeded while reading XML data. The nametable is a data structure used to store strings encountered during XML processing - long XML documents with non-repeating element names, attribute names and attribute values may trigger this quota. This quota may be increased by changing the MaxNameTableCharCount property on the XmlDictionaryReaderQuotas object used when creating the XML reader. Line 1, position 1298.

Any ideas on this?  I am new to ADX also.

Thanks

George