Can't export Telerik html5 Report (says "Loading")?

I have an MVC website which uses Telerik reporting with the html5 report viewer. The report displayed fine, but I couldn't export the report to any formats because when clicked the Export button just showed "Loading".

MySite/api/reports/formats also just showed

<Error>
<Message>An error has occurred.</Message>
</Error>

whereas it should show something like:

[{"name":"PDF","localizedName":"Acrobat (PDF) file"},{"name":"CSV","localizedName":"CSV (comma delimited)"},{"name":"XLS","localizedName":"Excel 97-2003"},{"name":"RTF","localizedName":"Rich Text Format"},{"name":"IMAGE","localizedName":"TIFF file"},{"name":"MHTML","localizedName":"Web Archive"}]

Solution

I solved the problem: it was due to my having added a method to the Reports controller (to create a dummy object datasource for a graph at design time). I found this out because on another pc, I got a more detailed error message from api/reports/formats, which included an ExceptionMessage XML item:

"<ExceptionMessage>/Multiple actions were found that match the request: ...

Moving the extra method from the Reports controller to a different one solved the problem.

I'm posting this in the hope that this helps anyone else with a similar problem.