My password is not accepted because it is too long
In password security, the longer the better. With a password manager, using more than 24 characters is simple. Unless, of course, the secure password is not accepted due to its length. (In this case, through STOVE.)
Possibly indicating cleartext storage of a limited field (which is an absolute no-go), or suboptimal or lacking security practices.
Sounds like they're using bcrypt. Feeding more than 24 utf8 characters into bcrypt won't do anything useful. You can permit longer passwords (many sites do) but they'd be providing a false sense of security.
Bcrypt is still secure enough and 24 characters are fine as long as they're randomly generated by your password manager.
The specification of the algorithm specifies up to 56 bytes, including a null terminator. If you're using UCS-2 (2+ bytes per character, like Windows, Java, Javascript, and more languages and platforms do), that's 27 characters (can't use the last half byte character pair). Add some margins for extended characters (emoji and such) and you'll end up just above or below 24. With UTF-8 you can end up doing much better (exclusively Latin-1) or much worse (exclusively non-Latin character sets). Verifying that on the frontend is a massive pain (string length in JS is unreliable) and dynamically switching codecs is a recipe for bugs and security leaks.
The 72 byte limit is the result of the internal workings of most bcrypt algorithms, but if you ever switch implementations you need to make sure that implementation doesn't change the internal workings if you rely on details like that. If the stars align you can use 71 characters (72 if you use Pascal strings), but that's far from a given.
Utf8 isn't ASCII. It takes up more space.
No, it does not take up more space for ASCII characters.
If you want a source, Wikipedia
the first 128 characters of Unicode, which correspond one-to-one with ASCII, are encoded using a single byte with the same binary value as ASCII
Good to know
I was just speculating