I would first make sure you have updated your EndNote to the latest version:
For X7 see
https://support.clarivate.com/Endnote/s/article/EndNote-X7-Updates?language=en_US
For X8 see
https://support.clarivate.com/Endnote/s/article/EndNote-X8-Updates?language=en_US
I would then use the following steps to clean up any document corruption:
If this does not resolve the issue then:
Please contact Technical Support directly.
1-800-336-4474 option 4 then 1 or
https://support.clarivate.com/Endnote/s/technicalsupport?language=en_US