The tools.wmflabs.org 404 page says:

Four hundred and four!
The URI you have requested, /xxx/, doesn't seem to actually exist.
Perhaps the webserver has temporarily lost its mind, or the link you've followed doesn't actually lead somewhere useful?
You might want to looks at the list of tools to find what you were looking for, or one of the links on the sidebar to the left. If you're pretty sure this shouldn't be an error, you may wish to notify the project administrators about the error and how you ended up here.

despite the fact there is no sidebar on the left

It should say something like:

Four hundred and four!
The URI you have requested, /xxx/, doesn't seem to actually exist.
Perhaps the webserver has temporarily lost its mind, or the link you've followed doesn't actually lead somewhere useful?
You might want to looks at the list of tools to find what you were looking for. If you're pretty sure this shouldn't be an error, you may wish to notify the project administrators about the error and how you ended up here.

Example page: https://tools.wmflabs.org/lta-tracker/