error NETSDK1134: Building a solution with a specific RuntimeIdentifier is not supported

[norgie]

Is there anything particular to take into consideration when building with .Net Core 5 SDK? I have a cake build script that runs just fine locally, but fails on our TeamCity build-server. The error we get is: error NETSDK1031: It is not supported to build or publish a self-contained application without specifying a RuntimeIdentifier. You must either specify a RuntimeIdentifier or set SelfContained to false.
If I do as suggested and specify a valid RID I get another error: error NETSDK1134: Building a solution with a specific RuntimeIdentifier is not supported. If you would like to publish for a single RID, specifiy the RID at the individual project level instead.
All the projects in said solution contain the following element:

<PropertyGroup>
    <TargetFramework>netcoreapp3.1</TargetFramework>
    <RuntimeIdentifiers>win10-x64;linux-x64</RuntimeIdentifiers>
  </PropertyGroup>

Tasks:

var taskBuildBackEnd = Task("BuildBackEnd")
    .Does(() => {
        InBlock("Building NOA Back-End", () => {
            foreach(var solution in solutions)
            {
                Information("Building {0} for .Net Core runtime: {1}", solution, runtime);
                var settings = new DotNetCoreBuildSettings 
                {
                    Configuration = configuration,
                    Verbosity = msBuildVerbosity,
                    MSBuildSettings = new DotNetCoreMSBuildSettings()
                };
                if (selfcontained) {
                    settings.MSBuildSettings.WithProperty("useapphost", "true");
                    settings.MSBuildSettings.SetRuntime(runtime); 
                }
                DotNetCoreBuild(solution.ToString(), settings);
            }
        });
    });

var taskPublishBackEnd = Task("PublishBackEnd")
    .Does(() => {
        InBlock("Publishing NOA Back-End", () => {
            CleanDirectory(paths.OutputDir);
            foreach(var solution in solutions)
            {
                Information("The target .Net Core runtime is: {0} with the selfcontained flag set to {1}", runtime, selfcontained);
                var settings = new DotNetCorePublishSettings
                {
                    Configuration = configuration,
                    SelfContained = selfcontained,
                    Runtime = runtime,
                    OutputDirectory = paths.OutputDir,
                    Verbosity = msBuildVerbosity,
                    MSBuildSettings = new DotNetCoreMSBuildSettings().WithProperty("CopyLocalLockFileAssemblies", "true")
                };
                if (selfcontained)
                {
                    settings.MSBuildSettings.WithProperty("useapphost", "true");
                    settings.MSBuildSettings.SetRuntime(runtime); 
                }
                DotNetCorePublish(solution.ToString(), settings);
            }
        });
    });

So this is looking like a Catch-22 situation. Before installing the 5.0 SDK on the build server, this cake script ran without a hitch (no element then). However, adding/removing the element makes no difference with the 5.0 SDK. Cake version: 0.38.5. My colleagues and I are completelly dumbfounded by this, so any help/pointers in the right direction would be highly appreciated.