Hi Quim,
Thank you for the details. The issue is simply the heavy caching, that did not populate the changes to the front-end. I have enabled this option to temporarily bypass the page cache: https://i.imgur.com/Adi3kdg.png
You can turn this off later, if you want to, once the cache is cleared.
Always make sure to update the plugin via the plugin manager (never via FTP), and clear all layers of cache after updating. In this case the jump from 4.12.2 to 4.15 was 17 releases apart, so I also recommend checking all the settings as well, to make sure everything is migrated correctly.
Also, reverting the plugin to an older version requires reverting the database as well. Reverting only the files may cause a corrupted database, and may require a complete plugin wipe, as the new data may not work with the older files.
I have renamed the old version to ajax-search-pro.old via FTP, you should delete that, once everything is working.