The clustered index IS the table. It has at the leaf level all the columns in the table. Hence you added a LOB column to the clustered index (which is why physical stats now shows a LOB_DATA allocation unit) hence the online index rebuild fails.
Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability