I too am having this problem.
I am using CC on iOS 11 on an iPhone. I have not tried Android yet. I am using Calibre 3.18 content server on linux. The issue does not exist with the first (default) library provided in the startup parameters. It exists with all others. I verified this by reordering the libraries in the service configuration.
What I am noticing is that for all other libraries I get a 404 error. Below is an example:
192.168.15.157 port-51165 - 08/Mar/2018:08:40:52 -0500 "GET /ajax/book/35cc5701-b2ed-4155-9174-82437f459c40?id_is_uuid=true HTTP/1.1" 404 197
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