Symptoms

Error: SQLite: unable to open database file

Cause

Training tasks/processes when multiple versions of document definition are applied to documents within the training batch with many documents present within this batch already.

Resolution

This error has no actual drawbacks and is fixed within the 12.0.3.4093 build and later, which can be downloaded from here.

In case you will update, please make sure to backup your data. 
The following articles can be helpful:

• • Creating an ABBYY FlexiCapture backup
  • Updating ABBYY FlexiCapture 12