"Username and password combination not found, or user account temporarily blocked"

If you receive this error message when logging in but your credentials are correct there could be several causes:

The Obvious Answer: A Typing Error

Don't worry, everybody does it!  Make sure you're entering both the username and password exactly as intended.  They're case sensitive so make sure they're upper case and lower case in the right places.  Check that you haven't typed the letter O when it should be the number 0, etc. 

Try typing the password into another application where you can see what you type, to ensure the characters being entered are the ones you intended to type.  It could be a keyboard locale setting altering what you type.  Just be sure not to leave the password visible in the other location, don't save it in plain text, and close the file or window/tab you typed it in when you're finished.

It may help to store the password in a password manager/safe (a programme on your computer that securely stores passwords) and paste it into your browser instead of typing it every time (this also has the advantage of allowing you to pick a longer, more complex and therefore more secure password, since you're freed of the need to memorise it).

An Invisible Character

If you're pasting the username or password rather than typing it (as recommended above) check that you're not also pasting an invisible character like a space or a new line character.  Again, you could paste it somewhere else then type something before or after it to show up and spaces or new lines.  As above, be sure not to leave the password visible in the other location, don't save it in plain text, and close the file or window/tab you typed it in when you're finished.

If Using MFA Ensure Your Clock Is In Sync

AppCheck's Multi-Factor Authentication solution uses Time-based One-Time Password (TOTP) technology.  This system depends on both parties' (yours and ours) clocks being synchronised.  They do not need to be in the same timezone, but the underlying time (in UTC) should be the same.  If your clock is more than a few seconds off after accounting for timezones this will produce an invalid TOTP, and you will not be able to log in.

To fix this, ensure your device's clock is accurately synchronised with UTC (this needs to be accurate to within a few seconds).  Some apps also have their own button to re-sync the time.