Hi Paul,
Hope you are doing good.
Do have a look at: SAP Note 155954(SAP Service Marketplace: Proxy address) .
Also check:
note 401196 Download Manager: Proxy configuration problems
and note 401195 Download Manager: Unable to read basket content
If the issue still persists, take a trace as per note 574885.
Hope this helps.
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Kind Regards,
Hemanth
SAP AGS
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