The topic "Build a Report Using Your Own SQL or MDX" in the Report Studio User Guide does not mention that IBM Cognos TM1 does not support reports that contain MDX added by report authors.
When MDX is added to a report using the MDX query object, and the data source is IBM Cognos TM1, an error message appears when the MDX dialog box is closed or the MDX is validated.
IBM Cognos TM1 does not support type-in MDX.
Resolving the problem
Do not add custom MDX to reports that run against IBM Cognos TM1 data sources.