Cannot register, always get: The e-mail address is invalid

- I've installed the forum (mylittleforum-2.4.20) at https://www.rapideuphoria311.com/forum/

- I've created my admin account (of course)

- I'm using PHP version 7.4 (should I try with version 5.6?)

- I've tried to register with other name/email (with/without spam protection),
but ALWAYS get the same error message: "The e-mail address is invalid".

I'm using a valid-real email address and valid registration details.

WHAT IS THE PROBLEM? Why can't I register to my own forum? It looks like a bug...

P.s. the my little forum looks just great!

Thank you,
shian

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Cannot register, always get: The e-mail address is invalid shian 2020-02-02, 04:36
- Cannot register, always get: The e-mail address is invalid shian 2020-02-02, 06:04
- Cannot register, always get: The e-mail address is invalid Micha 2020-02-02, 07:10
  - Cannot register, always get: The e-mail address is invalid shian 2020-02-02, 08:42
    - Cannot register, always get: The e-mail address is invalid shian 2020-02-02, 10:20
      - Cannot register, always get: The e-mail address is invalid Auge 2020-02-03, 07:55
        
        Cannot register, always get: The e-mail address is invalid shian 2020-02-03, 12:34
    - Cannot register, always get: The e-mail address is invalid Auge 2020-02-03, 10:00
      - Cannot register, always get: The e-mail address is invalid shian 2020-02-03, 12:38
      - Cannot register, always get: The e-mail address is invalid WorldofBB 2020-02-04, 10:55
        
        Cannot register, always get: The e-mail address is invalid WorldofBB 2020-02-04, 11:39
        
        Cannot register, always get: The e-mail address is invalid Auge 2020-02-05, 10:16
        
        Cannot register, always get: The e-mail address is invalid WorldofBB 2020-02-05, 12:01
        
        Cannot register, always get: The e-mail address is invalid Auge 2020-02-05, 09:19
        
        Cannot register, always get: The e-mail address is invalid shian 2020-02-05, 02:11
        
        Cannot register, always get: The e-mail address is invalid shian 2020-02-05, 04:21