We can create account with diacritic marks for example
Łukasz
or
żółć
But we can't login to this account using that login
You have entered an invalid username/password combination.
Reproduce:
Create account with
Łukasz
username
Try to login.
Did your password also include non-Latin characters?
No, password contains only Latin chars.
It has to be related to character encoding. Sometimes there are multiple ways of encoding the same character. As an example, Unicode includes precomposed characters for many Latin-based characters, but these same values can be generated using combining characters and a base character (combining cedilla + c = ç). The solution is to somehow normalize them.
Out of interest, what character encoding is the database table (mybb_users) using @Matslom?
I tested with MyBB 1.8.10 and it worked fine.
@Matslom can you ensure that mb_string extension is enabled for PHP?
Yep, actually I didn't have enabled mbstring extension.