It is likely related to this breaking change:
If you use
InMemoryAutomodels builder, or rely on Razor runtime compilation for editing templates via the backoffice, you need to reference theUmbraco.Cms.DevelopmentMode.Backofficepackage.If using models builder with one of the source code modes (that is, not
InMemoryAuto), you do not need to reference theUmbraco.Cms.DevelopmentMode.Backofficepackage. However, ensure the following settings are removed from your.csprojfile:<RazorCompileOnBuild>false</RazorCompileOnBuild> <RazorCompileOnPublish>false</RazorCompileOnPublish>If you use the RoslynCompiler class in Umbraco, you need to reference the
Umbraco.Cms.DevelopmentMode.Backofficepackage and update your namespace usings.