@ in URL breaks the Reporting API (no replies)

My URLs has the following structure:
/@alex
/@alex/post-1
/@alex/post-2

In the dashboard the data displays well, both for /@alex & /@alex/post-1 : http://cl.ly/image/0v1K2t3I0w2X

When I click export to JSON:


I get no results for /@alex/post-1 (results for /@alex are ok):


I guess something is wrong with the label parameter when querying for data:
Here is the label Piwik is using to get the data into the dashboard for /@korotaev/TGU2pvbC:

label=%2540korotaev+%3E+%40%252FTGU2pvbC

It does not work when querying the Reporting API. Tried different varieties of labels, still nothing.

Is there a special use of @ in Piwik?
Why do I get no results when querying via Reporting API?

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Here is some info to debug:
Successful internal request for data when loading Piwik dashboard:

Failed request to the Reporting API(empty results):