Some companies create many matrix-style reports using the SQLRS report wizard. When they use the report wizard, it uses Tablix Corner cells in the tablix report for any column on the report in the top left of the matrix that is not part of either row or column grouping for the report. When there is more than one Tablix Corner cell on a tablix for the report, the report will not load.
This was not working because PM Compass has not yet been updated to reflect a change that Microsoft made to the default report rdl schema (the definition for the rdl used for all reports).
Do not use the wizard, or alternatively, delete the default heading the wizard creates for the report and replace it with a new header to get rid of the Tablix Corner columns.