Due to recent changes made in Dropbox, users will likely experience issues with missing files in Engine Desktop 3.0. 

The fix for this is to use the recently released Auto Relocate files feature in Engine DJ. 

Simply right-click on one of the missing tracks in your library, then click Auto Relocate All as shown below. 

Once the process is complete, the files should be restored.

For further technical support, please open a support ticket.