XF 1.5 Urgent help needed (user import)

hello. So i managed to import everything from phpbb since we want to move on xenforo but our main board is having issues.

The import process completed properly. around 1.8million of users were imported among other stuff. But after import user rebuilding stopped around 350k and i get the following error: https://image.prntscr.com/image/3e6dqUlVQp_NQmV7R2Rg5w.png

Please assist me on this. Our board is useless atm. Looking for a swift resolution. Thank you