Have you collected filter logs to identify the failure?
I don't have an answer; but wondering if you are doing this in the midtier views or in Smart IT.
Actually after some research it seems that all user have the same problem. It has been narrowed to one thing:
In other content fields images may be added, and there is no problem with saving the Knowledgebase Data Entry.
It is not possible however to add an image in one of the Content Fields - In that case "Known Error" -> Workaround/Fix, which is as the third field of four available. Saving the title with image added is not possible, as well as adding the image to a Knowledgebase Data Entry with an Title created before (KBA has been saved, and then there was an attempt to add an image in the content field).
The question is how to modify the content dield so that images may be added there?