According to the releasenotes of Mendix 2.5.1 RC2 it requires to use the latest version of the Excel importer. However Excel importer v1.1.1 results into 7 (alike) errors: Form 'Column_NewEdit' Drop down 'dropDown2' Widget may not be required if the containing table row is conditionally visible. Maybe useful to change the Excel importer centrally instead that everyone has to change this on its own.
We are currently working on a fix for this. Expect a new version in the AppStore today.