I am having a similar problem. I am getting a 404 error when I try to visit my sitemap url. When I try to generate the sitemap, The bar does not get very far and the dialog box disappears quickly. I tried turning off third party add-ons, and that didn't work. I couldn't find anything about it in my error logs.
The fn_get_files_dir__path() should account for any compnay_id that might be in place. Note that if you are running a single store that has been upgraded from a pre-V4 environment, there will not be any company_id in the files path.
I ended up asking the cscart devs to look into it and they found that it was a problem with my app/functions/fn.catalog.php file.
I have a modification for displaying price ranges on my product category pages, and the developer that made it had changed the fn_get_products() function code. I guess I'll just revert to the original code whenever I need to generate a sitemap.