From what I've gathered, the only change made since the last build in Azure-DevOps is the version of this nuget-package. So either there is a mistake made in there (which I am not privy to investigate) or the problem lies elsewhere in the build task.
[error]f:\WorkB_tool\dotnet\sdk\5.0.102\Sdks\Microsoft.NET.Sdk\targets\Microsoft.NET.Publish.targets(237,5): Error MSB3025: The source file "C:\windows\ServiceProfiles\NetworkService.nuget\packages\package\version\staticwebassets\css\open-iconic\FONT-LICENSE" is actually a directory. The "Copy" task does not support copying directories.
The error is clear enough I suppose, but I haven´t found a resource on what is causing it or how to fix it.
By adding a file ending (css\open-iconic\FONT-LICENSE.txt) the build could proceed.
However, why this was suddenly an issue still perplexes me.