QB Issue Resolution:

Where is the company file stored?  If other than the main drive, try copying to the main drive and opening from there.

Resolution for Issue 'Unable to allocate the backup engine' available: Yes (Solved).
Source: Intuit Community forum.

Unable to allocate the backup engine: this 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 issue yourself by looking for a resolution described below. If it is a complex issue or you are unable to solve the issue, you may contact us by clicking here or by using other support options.

Support for backup engine
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Error Recovery for backup engine [Canada]
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