When logging in, you may notice an error that says "The submitted form was invalid. Please try submitting again."
If you see this error, it means you need to clear your website cookies.
The method to do this may vary slightly between browsers, but generally, this is how you do it:
In the address bar on the left side of the URL, there is a control that lets you see the website stability and view the cookies.
Click it, and on the resulting drop-down you can click the option that says "cookies and site data," and then where it says you have stored site data just click the delete/dustbin icon 🗑️
Then reload the page. It should let you log in normally without the error.
You can also do it through the settings page following this tutorial and deleting cookies for lmms.io:
Hope this helps you people with logging into the site if you can't!