Click the image icon, go to 2nd tab (Link) or 3rd tab (Upload). Select any image and press OK if on 2nd tab. On 3rd tab browse for any image, then a strange UI behavior appears with just scrollbars, now press OK.
In both cases after pressing OK it leads you to the first tab with an error message - Image URL missing. For some reason it doesn't take the image URL from other tabs to the first tab URL field
The issue can be reproduced also on your demo here https://www.responsivefilemanager.com/demo.php - the CKEditor section.
Click the image icon, go to 2nd tab (Link) or 3rd tab (Upload). Select any image and press OK if on 2nd tab. On 3rd tab browse for any image, then a strange UI behavior appears with just scrollbars, now press OK.
In both cases after pressing OK it leads you to the first tab with an error message - Image URL missing. For some reason it doesn't take the image URL from other tabs to the first tab URL field