QB Issue Resolution:

As I understand it only the premier accountant version can open two files at one time – and I am not sure that is true for non-us versions

hit F2 with QB open what does it say you have? you say you went from ’12 premier to ’15 premier yet the thread is tagged pro

Resolution for Issue 'I upgraded to QB 2015 Premier from 2012 Premier. Why can’t I now not open a second company while the one already open stays open?' available: Yes (Solved).
Source: Intuit Community forum.

I upgraded to QB 2015 Premier from 2012 Premier. Why can’t I now not open a second company while the one already open stays open?: 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.

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