This notification indicates that there is a file/ folder whose name contains an end character which is invalid on a Windows operating system. Such characters are e.g. dots (.) or spaces ( ).
Since the client synchronizes data from various devices, it may become an issue if the folder/ file is transferred to a device whose operating system can’t handle such end characters. Therefore characters like these should be avoided.
If this notification appears in the client, you simply have to rename the file/ folder and select a new name.