Try this link:

http://support.quickbooks.intuit.com/support/articles/SLN63972

and this thread:

https://community.intuit.com/questions/835762-error-code-6123-0

Resolution for 'convert 2 issue: 'I have installed QB Accountant 2016. My previous version was QB Pro 2008. I have several company files and have managed to convert 2 of them but the third won’t allow me!' available: Yes.

The convert 2 issue or error code is a known issue in Quickbooks Online (QBO) and/or Quickbooks. Support for this issue is available either by self-service or paid support options. Experts are available to resolve your Quickbooks issue to ensure minimal downtime and continue running your business. First try to resolve the convert 2 issue yourself by looking for a resolution described in the article. If it is a complex convert 2 issue or you are unable to solve the issue, you may contact us by clicking here or by using other convert 2 support options.

For more information about resolving issues related to convert 2 in Quickbooks/QBO or to get support for the issue 'I have installed QB Accountant 2016. My previous version was QB Pro 2008. I have several company files and have managed to convert 2 of them but the third won’t allow me!', click the link below: