Hi!
Thank you for the details, it helps me a lot.
Unfortunately the FTP is not working, I have tried FTP, FTPS and SFTP methods on numerous clients with no success, I get a connection timeout each time.
Luckily there is some information within the ajax response, the request fails at a cetain point, where the SKUs are about to be displayed within the titles. It looks like you use a custom code or a search theme modification for that, based on the ajax response output, I can’t tell which one though. The error is caused by that and because the taxonomy term search is also enabled. Because taxonomy terms does not have SKUs (nor any other custom fields), WordPress fails at that point. To resolve that you have two options:
1. To disable the search & return of taxonomy terms. I believe you are not using it anyways, as the grouping is enabled: https://i.imgur.com/h5V5iac.png
2. I can check the custom code, but since the FTP was not working, I was not able to. If you want you can upload it via a .zip file.
I hope this helps!