Sitemap not loading - Load error!

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I recently launched a website using the Framework theme, and I am using the C5 core version 8.3.2. I just discovered the Sitemap links in the Dashboard are broken. It was working fine for the first couple of months, but then, a couple of days ago I went to change something on the site map and can't access it. When I click on Sitemap it starts to load the page and then just displays "Load error!". If I use Chrome Inspector to inspect the term "Load error!", in the Console it reports

"Failed to load resource: the server responded with a status of 404 ()"

and the link shows

"https://www.brandontennis.org/index.php/ccm/system/page/sitemap_data?displayNodePagination=0&cParentID=0&siteTreeID=0&displaySingleLevel=0&includeSystemPages=0&_=1553791901695"

Through exhaustively searching the Forum for solutions, I did find a reference to this same issue and using the Framework theme on C5 ver 8.3.2 but in a later comment, they said this problem just disappeared and therefore thought it may be a cache issue. I have cleared site cache and browser cache but no improvement.

Does anybody have a clue for me on how to resolve this?

Pixelfixer
 
ConcreteOwl replied on at Permalink Reply
ConcreteOwl
If as you say it was caused by a cache problem, try the server cache..
Pixelfixer replied on at Permalink Reply
Pixelfixer
I think I have. I've cleared cache both in C5 edit mode, (server cache) and in Chrome (browser cache). And, enough time has passed that if it were cache related, it should have corrected itself by now. I have verified that the file 'sitemap_data' is where it should be, yet the error is reporting a 404 error, (file not found) and so, it appears the path being used is in error, but I can't seem to find where to correct this, or even begin to guess how or why it would have changed, since I don't think I had made any significant changes to the site, if at all, between the time it last worked and when I discovered it didn't. This really has me baffled. (not that that takes much. :-) )

Thanks for taking the time to offer a suggestion though, I DO appreciate any and ALL the help I can get.
ConcreteOwl replied on at Permalink Reply
ConcreteOwl
Your hosting company (server) may be running something like CacheWall or Varnish or a similar backend cache system, if you have access to cPanel take a look to see if they have a back end cache program running.
Pixelfixer replied on at Permalink Reply
Pixelfixer
I checked and verified there is no Server Cache service installed or active.