What I mean is, are all existing installs that have not been manually changed failing to launch, or is it only a subset of those installs?

To clarify something here. Everyone on the older version is as of right now facing the problem in T257384 correct? Only those of us who have manually intervened are able to run it?

A suggestion: Check if it's ={{PAGENAME}}= and remove in that case.

The reboot may have fixed some things. However I think there's now an error with https://wpcleaner.toolforge.org/install/getdown.txt which needs to be changed, it should contain:
appbase = https://wpcleaner.toolforge.org/wpcleaner/

It looks like it should be, but I of course haven't be able to run with it yet.

The issue mainly comes up with examples such as

{{Infobox person
| birth_date    = {{Birth date|1776|07|04|mf=yes}}<!-- {{Birth date and age|YYYY|MM|DD}} for living people supply only the year with {{Birth year and age|YYYY}} unless the exact date is already widely published, as per [[WP:DOB]]. For people who have died, use {{Birth date|YYYY|MM|DD}}.
| death_date    = {{Death date and age|2016|11|08|1776|07|04|mf=yes}}<!-- {{Death date and age|YYYY|MM|DD|YYYY|MM|DD}} (DEATH date then BIRTH date) -->

Where either due to a bad copy/paste or other issues the first comment is improperly ended, but the next line(s) contain valid and useful data that should not be commented out. I would suggest something along the line of if the line with the opening comment tag starts with \s*| then it suggests closing before the newline, else suggest before the next comment.

I would support having the option of ending on a line break or ending as current. It is usually within infoboxes, but occasionally tables, that the desired behavior is to close on the end of the line.