According to Intuit, Error Code -6000, -305 is caused either by a damaged company file (.qbw) or a damaged QuickBooks installation.
There are several troubleshooting steps discussed in the Intuit knowledge base article linked below. These steps include attempting to open a sample company file or restoring the company file that produced the error from a backup.
For more information on resolving Error Code -6000, -305, consult this Intuit knowledge base article.