I was beginning to think that the test code redirecting STDOUT had somehow cured the problem. I have been using the site for longer than usual without an error from the most used script - the one with the test code. I have an occasional error from other scripts.

But - I have just had a 500 error.
Looking at the logfile, the HTML belongs to the previously loaded page, not the one it is trying to load, and the timestamp shows it was the previous page.

So it looks like the 500 error is happening without the script starting up.

Again, I have checked the errors and there is nothing other than a few warnings. I have checked the RAM usage and it peaked at 664Mb out of 1Gb allowance over the last 24 hours.