When running multiple Web Applications under different Application Pools (with different Application Pool Users) you should make sure that those Web Applications have access to the MySite Web Application. If they don’t have access to the MySites this may cause issues when linking content to the Newsfeed from other Web Applications or when using social features provided by the Work Management Service Application (i.e. Tasks).

You’ll get the following error in the Application Log of the Event Viewer:
Log Name: Application
Source: Microsoft-SharePoint Products-SharePoint Foundation
Event ID: 3760
Task Category: Database
Level: Critical
SQL Database ‹Content_MySite› on SQL Server instance ‹SQLSERVER› not found. Additional error information from SQL Server is included below. Cannot open database «Content_MySite» requested by the login. The login failed. Login failed for user ‹DOMAIN\AppPoolUser›.

The solution is to run the following PowerShell command in the SharePoint Management Shell:
$WA = Get-SPWebApplication https://<URL to your MySite Web Application>