Thanks for reaching out. I tested this on the latest version of XD and Web Export plugin and it seems to be working fine on my end, so could you please test if it is happening with a specific file or multiple files? If you have already tried the basic troubleshooting steps, please contact the plugin support. To contact them right-click on the plugins name on the plugin panel and then click on support.
I would also double check that the file is there. If it is then, it might be naming issue in the html code. I would then check that the name and file paths are the same. But since this is a third party plug-in, any real solutions will have to come from them.