FIX transparent text input box

Have you tried looking at the same page on a different computer ? different browser ?
It is most likely a CSS problem, a change or oversight has left you with white text on a white background.
Suggest that you use a browser inspector and see what CSS is active in that box. You can experiment with css properties too. Then use the customiser to add suitable css to target this box’s class or id etc.

Nope, I’ve tried to change css to:

.actnbr-email-field {
	color: #000;
}

But nothing happened.
Other solutions? Maybe because I have the pro edition instead of business?

It’s not really recommended to use it unless absolutely needed but you could always try adding !important to try and force the style.

.actnbr-email-field {
	color: #000 !important;
}

This would overwrite any styles that are possibly set by a plugin.