This warning is showing you that a file or folder you have specifically selected has either been deleted, moved or renamed - and that these changes have not been updated in the client.

If the location has been deleted and it is not needed for backup anymore you will need to update the client. Login and navigate to the relevant backup set. Select "source" and go to "I would like to choose the files to backup". From here open up the location, ensure you tick "show files" so you can see the files it is trying to backup on the right. The file that is missing will show in red as show below.