Hmm...
Well your System Events log was chock full of Parser errors (almost 2000 pages of them!). They are all Too Many Forward attempt errors like this one:
« MODx Parse Error »
MODx encountered the following error while attempting to parse the requested resource:
« Too many forward attempts. »
SQL: The request could not be completed due to too many unsuccessful forward attempts.
[Copy SQL to ClipBoard]
Parser timing
MySQL: [^qt^] ([^q^] Requests)
PHP: [^p^]
Total: [^t^]
And your 500 Server Error page also remarks that there was an additional 500 error attempting to forward to an error document.
All of which makes me surmise that what is happening is that your pages are being considered off-limits to the public (despite the fact that they seem to be set up correctly as far as I can tell), which then forwards to your 401 page, which is also off-limits to the public, etc. And given that you can see these pages in the Preview window of the Manager (or in another tab of a browser that has a manager session open), this definitely seems like an internal MODx problem to me.
I’m going to keep sniffing around and see what I can figure out, but if any of you coders has any idea what might cause this please chime in so we can get it corrected.