Hi Pablo,
Thank you for the details, I was able to find the issue.
Due to the high number and content of custom fields, the field selector was malfunctioning. I’ve changed the query that parses the field names, and now it should run smoothly.
I’m going to re-work the custom field related selectors for the next release anyways, so you won’t have to worry about this when updating. For now, just copy over the wp-content/plugins/ajax-search-pro/ folder to the live environment, and it should work.