The "Path" field of the report action should be read only.
The "Path" field is not currently read only. It gives to the use the false impression that he can change this value and use a template stored elsewhere. But given #5808 (closed), users are prevented to open files outside of the trytond root.
Thus setting a invalid value for Path will result in an empty report template and a "Missing report file" error.
Given that the "Module" field of ir.action.report is read only, I think this field should be read only too.