I got this error during a cube process.
Errors in the OLAP storage engine: The attribute key cannot be found when processing: Table: 'dbo_facttable', Column: 'ColumnSK', Value: '2808'. The attribute is 'Column SK'. Errors in the OLAP storage engine: The attribute key was converted to an unknown member because the attribute key was not found. Attribute ColumnSKof Dimension: DimDimension from Database: SSAS_Cube, Cube:SSAS_Cube, Measure Group: FactTable, Partition: Fact Table Cases, Record: 1. Errors in the OLAP storage engine: The process operation ended because the number of errors encountered during processing reached the defined limit of allowable errors for the operation. Errors in the OLAP storage engine: An error occurred while processing the 'Fact Cases' partition of the 'Fact Cases' measure group for the SSAS_Cubecube from the SSAS_Cube database.
When I further analyzed it I verified that 2808 is in the dimension table. Since the CUBE storage is MOLAP, before processing the cube I have to process the dimensions. After that CUBE was processed successfully.