I experienced this same problem with JCE 2.8.18. With frontend editing of Joomla articles, when I tried to select an intro or full article image, the popup would show the error "index.php’s server IP address could not be found."
Additional details that might help troubleshoot the cause...
Checking the console, it reported errors:
Refused to apply style from
https://www.xxx.com/templates/yootheme/css/theme.11.css' because its MIME type ('text/html') is not a supported stylesheet MIME type, and strict MIME checking is enabled.
Refused to apply style from '
https://www.xxx.com/components/com_easyblog/themes/vintage/styles/style-5.2.4.min.css' because its MIME type ('text/html') is not a supported stylesheet MIME type, and strict MIME checking is enabled.
Refused to apply style from '
https://www..xxx.com/templates/yootheme/vendor/yootheme/theme/platforms/joomla/assets/css/site.css' because its MIME type ('text/html') is not a supported stylesheet MIME type, and strict MIME checking is enabled.
None of those files exist (EasyBlog isn't even installed). So it wasn't the MIME type issue, actually a 404 not found. I'm not sure where those files are being called from. They don't exist in the page code.
I disabled JCE and the errors were gone, so I know JCE was the cause. Installed JCE 2.8.19 beta 6 and problem is gone. Thank you!!