db2iupgrade may return error "DBI1124E Instance xxx could not be upgraded"

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After issuing db2iupgrade, if you receive "ERROR: DBI1124E  Instance db2inst1 could not be upgraded",
firstly ensure the instance is stopped by running db2pd - or checking db2sysc process(ps -elf|grep db2sys).
Next verify if you have issued db2iupgrade from the higher Db2 version where instance is upgraded to.
For example, if you are upgrading Db2 instance to version 11.1 from version 10.5, db2iupgrade should
be issued from version 11.1 product directory.

There is a scenario where the instance is stopped and db2iupgrade is issued from higher Db2 version,
however DBI1124E is still returned and in db2iupgrade trace(db2iupgrade -d), Db2 complains db2iupgrade is
issued from the same level of the instance to upgrade:

11113  |||||||| 1581569824 InstanceGlobalList::getInstanceInfo 20 -DATA-  , STRING = db2inst1 /opt/ibm/db2_v10.5 wse 10.5.0.7
11114  |||||||| 1581569824 InstanceGlobalList::getInstanceInfo 30 -DATA-  , STRING = db2inst1
11115  |||||||\ 1581569824 InstanceGlobalList::getInstanceInfo EXIT Tue Sat 1 11:11:11 2018 --  , INT = 0
11116  ||||||| 1581569824 checkInstanceVerRelforUpgrade 30 -DATA-  , STRING = 10.5.0.7
11117  ||||||| 1581569824 checkInstanceVerRelforUpgrade 40 -DATA-  , INT = 10
11118  ||||||| 1581569824 checkInstanceVerRelforUpgrade 50 -DATA-  , INT = 5
11119  ||||||| 1581569824 checkInstanceVerRelforUpgrade 60 -DATA-  , STRING = instance is at same level

In such case, this is likely due to your library path environment variable(LD_LIBRARY_PATH or LIBPATH) includes library path from
the previous Db2 level. To resolve the problem, remove Db2 library paths from the library path environment variable and reissue db2iupgrade.