2315396 - DB2 database instance upgrade fails with error "DBI1951W The instance "DB2<SID>" was not found in the instance list."

During a DB2 database instance upgrade on Windows, the following error appears:

The upgrade of the instance "DB2" has failed. The return value is "-100".

In the command prompt, the following error can be seen:

DBI1951W  The instance "DB2<SID>" was not found in the instance list.

Upon checking the db2iupgrade.log and db2iupgrade_local.log.* found in the tmp folder, the following errors can be seen:

1: ERROR:The upgrade of the instance "DB2<SID>" has failed. The return value is "2".
 
1: ERROR:An error occurred while configuring the instance "DB2<SID>".
 
Action <timestamp>: DB2CustomAction. Configuring DB2 instances
1: Configuring DB2 instances:.......Failure 
Action <timestamp>: DB2CustomAction. Configuring DB2 extended security
1: ERROR:SQL1390C  The environment variable DB2INSTANCE is not defined or is invalid. 

DB2 LUW on Windows

db2iupgrade, DB2INSTANCE , KBA , BC-DB-DB6-UPG , Upgrade SAP System , Problem