Upgrading to Joomla 3.9.13 and sh404SEF 220.127.116.1132
I upgraded to Joomla 3.9.13 and had problems with four of six sites giving the white screen of death. At the time of the upgrade, I also upgraded several plugins, including Akeeba Admin Tools, Akeeba Backup, OSMap and sh404SEF. The two sites that did not have problems did not have sh404SEF. I turned on Joomla and PHP debugging, and quickly learned that a fatal memory error was occurring, so I increased the maximum PHP memory allocation from 32M to 128M and all problems went away.
If you have problems with the white screen of death in upgrading to Joomla 3.9.13 or sh404SEF 18.104.22.16832, check your memory allocation first to see if this fixes the problem.