While working with the Hyperion Essbase Spreadsheet addin to retreive information from an accounting server, the error message "Currently, multiple reports per retrieval is not supported" started to show up, on reports that previously had worked without any problems.
And other users could use the same reports to retreive data without any problems.
After much tinkering and testing I finally found the culprit: the Excel application was open with multiple instances.

You can determine this by opening Task Manager (press Ctrl+Alt+Del and choose Task Manager).
If you look in the Processes window, then you will probably find multiple instances of EXCEL.EXE, or maybe only one even if you have closed your visible instance of Excel.
Using Task Manager you can also close those hidden instances of Excel by clicking on the process name and then click on the End Process-button in the lower right corner of the dialog.
Restart Excel and your Hyperion report should now work as it was intended to.