Error when upgrading from Umbraco 13.7.2 to 13.8.0

Questions
1

Upgrading in Visual Studio from 13.7.2 to 13.8.0 gives no error, but when I try to build the solution I get this error:

InvalidOperationException: Sequence contains more than one element
   at System.Linq.Enumerable.SingleOrDefault[TSource](IEnumerable`1 source)
   at Microsoft.AspNetCore.StaticWebAssets.Tasks.GenerateStaticWebAssetsDevelopmentManifest.<ComputeManifestAssets>d__19.MoveNext()
   at System.Linq.Buffer`1..ctor(IEnumerable`1 source)
   at System.Linq.Enumerable.ToArray[TSource](IEnumerable`1 source)
   at Microsoft.AspNetCore.StaticWebAssets.Tasks.GenerateStaticWebAssetsDevelopmentManifest.ComputeDevelopmentManifest(IEnumerable`1 assets, IEnumerable`1 discoveryPatterns)
   at Microsoft.AspNetCore.StaticWebAssets.Tasks.GenerateStaticWebAssetsDevelopmentManifest.Execute()

Any ideas?

2

I had the same problem. It turned out to be down to Umbraco Engage, which was creating an extra folder in wwwroot/App_Plugins. When I excluded this folder from the solution, the project built fine. I think the latest version of Engage no longer creates this folder.