The error typically indicates the app pool was recycled during this event. Could you please check the event log to see if you see any STARTAPP, or ENDAPP Event codes around the time you attempted the import? Also, is it possible to enable the debug mode and send us the full error details?
If you are receiving the start/endapp during the import process the import will never complete. The application pool is outside the control of Kentico, so you will need to find a way to prevent the app pool from restarting during the import. You could try assigning a separate app pool to this application if you are sharing the app pool with another application on the server, or you could also try running the import when the site isn’t under a high load.
Best regards, Roman Konicek