EndNote warns about about storing EndNote files in cloud-syncing folders (EndNote libraries subject to corruption). See the Important note for macOS users (middle left of page) and the General library safety notes (bottom left of page) for Mac and Windows users.

Beginning with EndNote X7.2x, users can share their EndNote library or library group with collaborators. Each collaborator must have or set up an EndNote online account.

EndNote and the Track Changes functionality in Word do not always play nicely together. To avoid issues with duplicate in-text references, make sure that the Track Changes option is set to show "Final" (NOT "Final: Show Markup"). Alternately, turn off Track Changes completely.