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You AR Server Licence is expired . before saving the transformation can you enable the AR API log you will get the exact error.
Thanks & Regards
Thanks and It worked, licensing was the issue.
Just for people drawn here by the exhilarating headline, not the complete error message, this can also be due to a multiple match for the datasource name in UDM:Repository:Database. This is something that can bite you if you run arsrename (at least that is what I think happened to me).