After I configured Let’s Encrypt on a CentOS server. I then configured my Magento 2 installation to use HTTPS on the admin side. Following these changes I could not import CSV files into Magento. The import process would just hang at “Please Wait” while doing a data check on the import file.
I opened the developer tool in Chrome and the console was showing this error:
in a frame because it set multiple 'X-Frame-Options' headers with conflicting values ('deny, deny, SAMEORIGIN'). Falling back to 'deny'.