Can't call controller method on dotnet core api: no middleware to handle the request

I'm facing an issue with my dotnet core api with angular. I simply made a login form and a UserController to handle login requests with jwt but whenever i hit the "Login" button and call the UserController's Login method from my angular app with this:

this.http.post<any>(environment.baseUrl + `api/user/login`, user)

I always get the following message:

"The SPA default page middleware could not return the default page '/index.html' because it was not found, and no other middleware handled the request."

I'm looking for more than a solution on this one because i've been searching to fix it the whole day and i couldn't figure out how i could solve it. I looked at iis logs, eventviewer logs. I tried debugging and did a fair amount of internet research, but I can't find a proper way to actually handle this kind of error

Since the error is raised by the dotnet core framework, i don't really know how to determine if it's my controller that is broken or if i made a mistake while configuring services.

How do you usually figure this out? Thanks for your time and your help in advance ;)

Startup.cs:

public class Startup
{
    public Startup(IConfiguration configuration)
    {
        Configuration = configuration;
    }

    public IConfiguration Configuration { get; }

    public void ConfigureServices(IServiceCollection services)
    {
        var ConnectionString = Configuration.GetConnectionString("Default"); 
        services.AddDbContext<TheCompanyContext>(options => options.UseSqlServer(ConnectionString));

        services.AddControllers();
        services.AddSpaStaticFiles(configuration =>
        {
            configuration.RootPath = $"ClientApp/dist";
        });

        services.ConfigureCORS();

        var key = Encoding.ASCII.GetBytes(Configuration["AppSettings:AuthenticationSecret"]);
        services.AddAuthentication(x =>
        {
            x.DefaultAuthenticateScheme = JwtBearerDefaults.AuthenticationScheme;
            x.DefaultChallengeScheme = JwtBearerDefaults.AuthenticationScheme;
        })
        .AddJwtBearer(x =>
        {
            x.RequireHttpsMetadata = false;
            x.SaveToken = true;
            x.TokenValidationParameters = new TokenValidationParameters
            {
                ValidateIssuerSigningKey = true,
                IssuerSigningKey = new SymmetricSecurityKey(key),
                ValidateIssuer = false,
                ValidateAudience = false
            };
        });

        services.AddAuthorization(options =>
        {
            options.FallbackPolicy = new AuthorizationPolicyBuilder()
                .RequireAuthenticatedUser()
                .Build();
        });
    }

    public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
    {
        if (env.IsDevelopment())
        {
            app.UseDeveloperExceptionPage();
        }
        else
        {
            app.UseExceptionHandler("/Error");
            app.UseHsts();
            app.UseHttpsRedirection();
        }

        app.UseStaticFiles();
        app.UseSpaStaticFiles();
        app.UseSpa(spa =>
        {
            spa.Options.SourcePath = "ClientApp";
        });

        app.UseRouting();

        app.UseAuthentication();
        app.UseAuthorization();

        app.UseEndpoints(endpoints =>
        {
            endpoints.MapControllers();
        });
    }
}

UserController.cs:

[Route("api/[controller]/[action]")]
[ApiController]
public class UserController : Controller
{
    public UserController(IConfiguration configuration)
    {
        Configuration = configuration;
    }

    public IConfiguration Configuration { get; }

    [HttpPost]
    [AllowAnonymous]
    public ActionResult Login([FromBody]User user)
    {
        // do login stuff
    }
}