One of the most consistent causes of Endnote errors is out-of-date software. Often this comes from a concern about losing data, or rumours that updated software has compatibility issues.
Actually, software updates are designed to stay ahead of compatibility issues - particularly with Microsoft Office365 issuing more regular and smaller feature releases rather than giant updates. Software updates address security vulnerabilities, software bugs, and add new features over time. The easiest way to manage this is to switch on automatic updating - whether that is for MacOS, Microsoft Office, or Windows.
Endnote also does updates, for example X20 is a new upgrade to X9. These kinds of big updates can't be automatically scheduled, and users need to kind of 'opt-in' by requesting the update. This is worth doing.
In the past, advice has been to uninstall the old Endnote before upgrading to the new version, but this isn't essential. To ensure the safety and compatibility of Endnote Libraries (especially if they aren't backed up online), we recommend installing the new software, checking that everything works fine, and then uninstalling the old software to keep the system clean. Of course you can create a backup of your Endnote Library before doing this if you wish, just to be safe.
Most compatibility bugs in Endnote can be resolved with a software update.