The error usually appears if there a peer with Selective Sync mode in mesh. It means that one of the peers informed others about some new or updated files but by the time other peers got to actually downloading it, the file had been removed.
For example, a peer with Selective Sync enabled has a new file in the share. It tells other peers about the file. It may take a while for others to actually recheck the file, merge the folder tree before they get to actually downloading it. By this time the file is removed from source peer and has only placeholder instead. So other peers won't have the actual file to download, and give this error expecting that it will appear some time later, but in reality it is a "ghost file" that no one has anymore.
Clicking on the warning one can see the filenames.
If the files don't exist anymore, you can press the Ignore All button to hide the warning. One can also disable this warning via power user preferences:
If the files exist, one should be careful - may be the more up-to-date version actually exists on a certain peer which is offline at the moment. If you are certain that local version of files is the most up-to-date, touch them or move them out of a shared folder to some temporary location and then move them back to the shared folder.