You can try logging in once with your android instead of your pc, no need for the app, just log in from the site and see if you can leave a comment that way.
If you can, the problem is with the pc/browser. If you still can’t, then it’s an issue with the account. Anyway, I’m also a problematic one when I need to install apps to test, try etc hahaha
Another thing, I had some problems posting comments when I registered. I changed my username (the one in the settings that you use to register, not the display name), and everything worked fine. This was before mdl created the new “news” site, so it might be unrelated, but you could try that as well.