Symantec Endpoint Protection Manager (SEPM) Upgrade stops at initializing and fails. Subsequent attempts to upgrade fail due to connection error. You identify the following symptoms;
The SEPM upgrade process is supposed to set the database to SINGLE_USER mode initially and then set it back to MULTI_USER mode once the process is complete.
The inability of the upgrade process to set it back to MULTI_USER mode can happen if there is something already interacting with the database when the upgrade is initialized (when the database is set to SINGLE_USER mode). Thus not allowing the upgrade to proceed.
If the issue is present, then it will be necessary for one to go into the SQL Server and manually set the SEPM database to MULTI_USER mode. Once back in MULTI_USER mode, the upgrade should proceed as normal.
To prevent this from occurring, before initializing the upgrade, ensure that all SEPMs on site have been stopped. Then also make sure that any 3rd party tool or entity which interact with the SEPM database are also stopped or not interacting with the database.