Well sometimes just browsing these forums inspires a solution.
One installation of this script has been working for 2 years and suddenly all events stopped displaying, including old ones that had been fine before.
After digging around for an hour I found the events.txt file was fine but that schedule.txt was completely blank. Not even a header row.
It appears that the installation server had a disk quota system in place with no scheduled means of deleting old log files. After deleting about 150 MB of logs, the events began reappearing. Because schedule.txt was blank, all old events were completely gone, even though they were still in events.txt.
So we want to blame the ISP. The fact is that text databases are hard on a server, they're clunky as a Monty Python dirigible, and when dealing with them there should be some form of testing in the programming beyond simple permissions to make sure we can write the file without producing an empty file. This script is not even returning errors under these conditions. It just says "Event Added Successfully" with no indication you've just completely killed the calendar history for the last two years.
Just a FYI, I know it's just two cents.
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