It seems your previous account was linked to a different email than what you tried to log in with. It was registered a long time ago with a @volny.cz email address, and has a bunch of posts.
Jirka Nejedly wrote:Surely I cannot sign up with my previous mail
Yes, but if you try to do that, the system would prompt you to log in with that email.
When you tried to log in / sign up with your @seznam.cz email, it created a new account for you, because we never had an account with that email.
And you have also signed up to another account with a @mikros.cz email.
If you can show us some proof that the first account belongs to you, we can probably merge all three accounts for you.