Smartcrawl sitemap error causing problems

The sitemap shows this error in a browser

XML Parsing Error: not well-formed

Location: https://MYDOMAIN/sitemap.xml

Line Number 11, Column 39:<loc>https://MYDOMAIN/?p=1973&preview=true</loc&gt;

Also when I navigate to the location

/httpdocs/wp-content/uploads/sitemap.xml

the sitemap .xml file and also the sitemap.gz.xml file cannot be seen, but they reappear after requesting them in a browser, but of course I have the error above.

I often also see “File does not exist” in logs from Googlebot.

My question is therefore why is Smartcrawl making these errors, causing the parsing error, the sitemap not being in the uploads directory on every occasion I look, and then the files to be missing from Google?

It seems very hit and miss whether or not the sitemap is actually available when Googlebot comes looking.