Thank you for pointing out my mistake, @aeromarine.
There was some accidental cross-pollination between my development files and the demo files.
In case you are wondering, the intention is to leave those references in /include/client/footer.inc.php but to have two empty files:
osta/inc/back-button.html
osta/inc/database-reset-warning.html
It sounds like you have already fixed the problem on your end, and it is a perfectly good solution so kudos to you for figuring it out and sharing your solution.