Erro date

Sounds like something is interfering with the calendar picker and the text field (changing the – into /’s).
Probably another plugin dealing with dates.

I already found it is in conflict with this thema: https://themeforest.net/item/keylin-wordpress-magazine-and-blog-theme/29954407 it is original

Aha! That’s good progress.
Unfortunately it’s a theme forest theme…

Surely the theme maker can figure this out and provide an update to the theme to fix the issue ??