1. Issues detected after submitting sitemap

Enable XML Formatter in the Extension setting to debug and resubmit the sitemap index file. You can easily find which xml tag or which line is causing the issue when XML formatter is turned ON in the extension setting.

1.1 Invalid URL issue

1. Google Search Console displays the line where the issue is detected. Note that line number
2. Open the respective sitemap link in a new tab of your browser
3. Right-click on the page and click on view source code
4. Go to the line in the source code page and here you can find which link is causing trouble

Most of the cases the tag will be empty. In such cases you need to fix those pages. For example if you see the following, it means the image you added to that page is not a valid image or that image doesn't exists in your server. You might need to edit that page and update the image. 

<url>
        <loc>http://yourdomain.ro/some-keyword</loc>
        <changefreq>weekly</changefreq>
        <priority>0.7</priority>
        <image:image>
            <image:loc></image:loc>
            <image:caption>some caption</image:caption>
            <image:title>some title</image:title>
        </image:image>
    </url>