Yes, I did try to enter e-mails of few domains (gmail, yahoo, rapideuphoria311.com).

All e-mails, including @rapideuphoria311.com are using only (7-bit) ASCII characters (I assume that digits 0-9 are included...).

I've also cleared my browser's cache completely, and restarted it (Firefox).

The installation of the forum was easy and fast, and everything else works...!

Are there any restrictions of length of passwords, database name, etc?
Are the characters '_'; '-'; '0-9'; 'a-z'; 'A-Z' can be used for all purposes?
In the "phpMyAdmin" there is a setting for "Server connection collation": 'utf8mb4_unicode_ci'
The database name is: 'rapideup_mylittleforum-2.4.20'
Database user name has '_' in it.

P.s.
I'm not a PHP programmer, I can only follow instructions as needed.
Therefore I cannot really look at the code by myself.

Thank you, shian

OK - I will delete the forum and create it again...