I never saw anything like that. So I can only suppose that your accounting database might be corrupt. And deleting it might resolve the problem.
Data is stored in files /var/adm/*tmp*. For more info: man utmp
I think some of the files may be deleted (or better: truncated to 0 bytes) without any problems, but I don't know details.
Warning: Read the manual, and delete at your own risk.
hope this help