Hello,
I discovered a bug in the lab system's epub export. While learning for the first oral exam I discovered some epubs don't work at all*, namely "TCP/UDP" and "The Basics". Because the exam is over now I had some free time to dig into the issue. I found out that the escaping of the 'ü' in moepi's last name ("Günther") in the book.opf somehow causes this problem, replacing it for example with "Guenther" leads to a working epub (at least Okular opened it, I didn't try with the others). I hope this information helps you to find the error in the system and fix it.
If there are further questions, please do not hestitate to contact me!
Best regards,
Michael
P.S.: students like PDF files, too ;)