Another cryptic message by Encore CS6.
"frame values not permitted by indicated format".
As it happens the culprit in my case is the caption file.
The film asset is 23.98, Encore conforms the timeline to the asset.
The caption file (.scc) works fine with a 29.97 Timeline and asset, but it generates the error on a 23.98 Timeline/asset.
Fix: I imported the .scc into Annotation Edit set to 23.98 and exported it to .scc. The roundtrip worke, the exported file did not generate the error.