Ok, problem solved (thank you Steve-O!): The book was stuck on a very old version (2011someting), and never upgraded: Even though we always pulled the code directly from moodle.org, and neither the upgrade to Moodle 2.4 and then 2.7 displayed any Errors, the 2011something-version stayed. However, problem solved now.
Book
Several issues with book on Moodle 2.7(.1)
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