I’m sorry for your trouble!
There are two ways to fix this. Either download and install osTicket-Awesome-1.10.1-v1.4 again (it has been fixed), or simply create two empty files:
osta/inc/back-button.html
osta/inc/database-reset-warning.html
If those two empty files are in place then PHP won’t spit up any more errors.
In case you are wondering, those files are used in the DEMO version but are intentionally blank in the official release.