Thank you so much for getting in touch with us, and our sincere apologies for the inconvenience.
The DOMDocument will issue this warning if there is invalid HTML.
Could I kindly ask if you would be open to submit a support a ticket here -> https://www.wpgmaps.com/support/ -> and also perhaps be open to provide us with FTP access so that we can take a closer look as to what is causing this issue?
We will then be able to debug this further and get back to you as soon as possible.
Could I also kindly ask if you could please confirm the WP Google Maps versions installed on your site?
Again, we do apologize for the inconvenience.
We look forward to hearing back from you and resolving this issue ASAP.