Users with R12.3 desktop client is build 3623 are unable to connect to DE (dSeries) R12.2 build 2394.
If an older DE version (e.g 12.2 to 12.3) was upgraded and then restored back then the DB may still have new version property data. The tracelog will show entires like these:
deserver.exampl.com:7500)] [receive] - CTOKEN(80000000) VERSION RESPONSE BUILD_VERSION(R12.1) VERSION_FROM_DB(12.3.00.00) BUILD_ID(2394) BUILD_DATE(202005041800)
2021-10-13 10:25:47,165 trace [CA_NONPROD (deserver.exampl.com:7500)] [receive] - CTOKEN(A0000000) 03 000000 ESPWSS0000E Cannot create command CLIENTVERSION 12.3.
2021-10-13 10:25:47,176 trace The command channel [deserver.exampl.com:7500] has been closed.
Release : 12.3
Component : Workload Automation ESP dSeries
When an older version is upgrade and reverted back the DB may still have information about new release. This can result in Desktop Client version not being properly recognized and will result in disconnect. Use the older version of Desktop Client to connect to to DE server. When backing out of an upgrade, always take back up of database and restore to pre-upgrade state.