This is the only problem I currently have when using this addon, but hopefully that will be resolved with the PR mentioned above where there will be a before and after script.
A basic example is from your TV Guide addon LibreElec restricts browsing to this addons userdata folder where the guide.xml is produced, the same restriction occurs with the TV HeadEnd addon.
You just need to enable "Show hidden files and directories" from Kodi "Setttings\Appearance\File Lists".
Make sure "Settings Level" is at least "Standard".
Then you can point to "/storage/.kodi/userdata/addon_data/service.webgrabplus/guide.xml" in TV Guide Fullscreen.