Hello @hube2
I found the solution here :
After many tries I found the solution based on this answer : https://wordpress.stackexchange.com/questions/307794/error-in-validate-field-with-acf-plugin-in-wordpress#comment473230_319945
I called
if (!is_admin()) {
Problem comes because of ajax use and is not admin function.
Hope this help someone else 🙂
