I had the same problem: build and rebuild didn’t seem to do anything. So I downloaded the sourcecode and tried debugging the cause. Turns out the very first lines of DoBuild cause the problem:
The ArgumentException is not caught and hence nothing happens. Replacing the try-block with “encoding = Encoding.UTF8;” fixed the problem for me.
Since I’m pretty new to monogame I don’t really know what to do with this information except post it here - hope somebody can use it to fix it appropriately.