Hi,

Environment: latest ossclass + latest gamma theme

If the "replace contact seller" option for plugin is ON, the Contact Seller button inside a post will have the standard form replaced with: Phone + Message (required)

If phone field is empty, i get error: "Invalid email address.Message: this field is required"
If phone is not empty (any value), i can submit

@sebastian
This does not look to be latest gamma version.
You may try to change jquery version in Settings > General.

I guess I wasn't using the latest version of my theme. I updated the theme (gamma from 1.4.2 to 1.5.0) and now it works

Thank you for your response

@sebastian
Glad to hear that