What is an "Invalid filename" syncing error?

If you are a Windows user and you see an Invalid filename syncing error in your Koofr desktop client, you should rename the files you are trying to sync, so they do not include characters that are incompatible with the Windows file system (< - less than, > - greater than, : - colon, " - double quote, | - vertical bar or pipe, ? - question mark, *- asterisk, . - period, emojis or a space at the end of a file/folder name). Please note that \ - back slash and / - slash are also forbidden on all platforms.