The problem is caused by the mismatch between the version of the Microsoft.AspNetCore.Components.WebAssembly and Microsoft.AspNetCore.Components.WebAssembly.DevServer packages referenced by the blazor project, and the version of the .NET SDK. In the ABP template, these packages are referenced with a wildcard version (5.0.*), so the latest matching version (5.0.9) was restored; but I didn't have .NET SDK 5.0.9 installed (my latest was 5.0.3). Installing .NET SDK 5.0.9 has fixed the problem.
WIth the problem above fixed, the app can run now, but the blazor website does not work because the file /_framework/blazor.boot.json cannot be found (404). It seems this file was not generated at all at build time.