After upgrading from 5.13.2 to 6.0.3 swagger-ui is not working

[mythz]

This request looks in conflict with the new pre-defined /api route which should only be used for ServiceStack APIs if enabled.

See this previous answer for how to change and disable the /api pre-defined route:

New pre-defined routes

Info about the New /api route. Is /api hard coded into the [Route] path? as it shouldn’t be registered if your App is hosted on a custom path. Can be changed with: ConfigurePlugin<PredefinedRoutesFeature>(feature => feature.JsonApiRoute = "/custom/{Request}"); Or prevented from being registered with: ConfigurePlugin<PredefinedRoutesFeature>(feature => feature.JsonApiRoute = null);