Cannot register, always get: The e-mail address is invalid (General)
Now it's OK. (It works!).
The problem was that I was using PHP version 7.4;
When I switched to PHP version 5.6 and then installed everything from the beginning - I could register immediately a new user without any error.
Now I'll continue to use the forum and hopefully everything will work fine... let's see.
Thanks a lot, shian.