Try running a rebuild. That may fix it.
File menu > Utilities > Rebuild
If not backup your file […]
Resolution for 'backup your file issue: 'Can someone explain why my balance sheet suddenly became out of balance?' available: Yes.
The backup your file 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 backup your file issue yourself by looking for a resolution described in the article. If it is a complex backup your file issue or you are unable to solve the issue, you may contact us by clicking here or by using other backup your file support options.
For more information about resolving issues related to backup your file in Quickbooks/QBO or to get support for the issue 'Can someone explain why my balance sheet suddenly became out of balance?', click the link below: