This error usually occurs when a route from an older version is loaded in to a latter version. Thumbnail is the old tag and thumbnails (plural) is the new tag.
What stagecoach says will work but a better solution is to replace the line/tag that says "thumbnail" with the lines below :
thumbnails
{
0
{
image "thumbnail.jpg"
width 240
height 180
}
}
thumbnail.jpg is just by way of an example and the file name can be anything you want and can be from the original folder if an image for the route is already present (change name to that in the folder) or a screenshot you have taken in game and resized and named appropriately.
Doing it this way will show an image for the asset in CM and in game in the route list when the route is selected.
