QB Issue Resolution:

log out of QBO, clear the cache and cookies from the browser, close and reopen the browser and log back in to see if that helped

Resolution for Issue 'When I click on the Customers link on left hand side, it’s not working. Any advice?' available: Yes (Solved).
Source: Intuit Community forum.

When I click on the Customers link on left hand side, it’s not working. Any advice?: 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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