I have been wrestling with this issue for 3 months now, and just yesterday finally figured out the issue.

To start off, I was having issues logging into the Official Minecraft Website and the Minecraft Launcher itself. They kept telling me that my email/password was invalid. But the confusing part was that I was able to log into the Mojang Account website with no issues. So I knew my login info was correct.

It took forever to figure it out, but apparently, the modern Minecraft login systems break when a user's password begins with a special character whilst the classic login systems don't. I've tested ":" "/" "&" and "%", all produced the same issue. I'm not sure why this is the case, but the moment I changed my password to something that starts with a normal character, it let me in with no issues.

I hope that this bug report can help someone else who likes to begin their passwords with special characters.