If the replication is not flowing to the Export queue, then the error can be due to corruption of checkpoint files located in /$SP_SYS_VARDIR/state directory.
Try the following to see if it works:
1.Shutdown Shareplex,
2.Rename the shs_chkpt file
3.Restart Sharpelex
If the above does not help, please contact Support for further troubleshooting.