I am experiencing exactly the same problem after updating Joomla to 5.4.7 and Helix Ultimate to the latest version.
When I try to remove the featured image from the Blog Media tab, the preview disappears, but I receive the message: “Unable to remove image. Please try again.” After saving or reopening the article, the image is still assigned.
This is very irritating, especially when copying existing articles and replacing their images. Is there any temporary workaround or file/database fix? Please let us know how this issue can be resolved and whether an update is planned.