This message doesn't sound familiar to me at all and I quickly searched the source code but can't find it either. Though it could be an error coming from the ehcache library that is being forwarded.
A quick search on the string "Image not available. URI:" on google reveals that it might also be coming from the Apache FOP library. We're using this when generating PDF files but I haven't seen this error before. Are you also using the FOP library in a custom implementation? If not, can you verify that the errors occurs when you are generating documents? It could also be coming from another custom Java action/library that is used in either an appstore module or your own implementation and that just happens to have an overlapping exception message.
Hope this helps a little in finding the cause.