Thank you very much! I did as you advised and noticed/fixed the problem.
SDL Support wrote:
The message is simply telling you that you have some tags in your target that are in a different order to the tags in your source. This might be responsible for problems when back converting the document so it is important to check this.
If you can't see them it is possible that Studio has hidden them, particularly if they are formatting tags. The easiest way to check this is to use ctrl+shift+H and this will toggle the display of hidden tags on and off.
Maybe this will help you to see where the difference lies?