Check out the following links:
Upgrade from 7.11.12 to 7.12 does not work · Issue #9592 · salesagility/SuiteCRM · GitHub
Unable to upgrade to 7.11.1 - #5 by chamullo
Steps to Upgrade SuiteCRM 7.11.13 to 7.14.6
How to upgrade your SuiteCRM installation - PlotHost
Upgrade from 7.11.18 to 7.11.22 fails and breaks the frontend and login!
7.11.10 to 7.11.12 upgrade wizard immediate 500 Error - #16 by dsoden