[Koha] Installation gives nondescript 500 error

[C.S. Hayward]

I had, I thought, run through the instructions to install Koha. Now, when I try to connect to the library by hostname, it simply says "Internal Server Error". There is no additional information in any of the /var/log/apache2 error logs, and no additional lines in /var/log/koha/library/opac-error.log.

What can I do to get useful diagnostics and step closer to getting things working?

Thanks,
Br. C.S. Hayward, https://cjshayward.com