Are you importing as part of an upgrade or just from one system into another?
What are the Thingworx platform versions of the system the entitites have been exported from and of the system they are being imported to?
What extensions were installed on the original system? Are the extensions on the new one matching the original?
If you are not in the U.S, when/if theere was a daylight time change ?
I addition to what Polina asked; please check Application logs of the Timeframe when error occurred to get more details on the error.
What are the exact Steps followed to take the export? And what are the Steps followed for import?
Were all the extensions from the previous system imported in new system before importing the entities/data?