With EndNote X9.3 a new library format was introduced. This format is not compatible with earlier versions of EndNote.

Opening an older library with X9.3 or 20 will create a new, 'converted' library e.g. 'My EndNote Library-Converted'.

If you use online sync, the name change will make it look as if you are synchronizing a new library.  To avoid problems either move the old library, then rename the converted library to the older name; or force sync to run immediately after conversion - it will merge the two libraries (the online library and the new 'converted' library) which should be fine because they should have the same content

Online Sync will also work with both versions of the library so if you have a post X9.3 version on one machine, and a pre X9.3 version on another, you can use Online Sync to keep the libraries the same.

For more details refer to this Clarivate site