> entered "1" and got an error (as expected)
Yes, but please do have a close look at the screenshot "server-side1.PNG" you uploaded yourself.
It shows exactly the problem that I mentioned right from the start:
"Server side validation works in principal, but spits out an incorrect message when the validation (correctly) fails: "Enter 0 or more characters, not fewer.""
You might just as well call that message funny or stupid, but it will definitely confuse users when they are told not to enter fewer than zero characters.
> I noticed it has changed (the required is removed, when clicking the checkbox)
Yes, the icon is removed, but try to submit with an empty value and validation fails and tells you you must enter a value.