2626319 - SUM Error "Error during unregistering func" during phase RUN_UCON_REGISTER_FUNCS_INIT of Upgrade

During an upgrade or add-on installation, SUM halts at phase PREP_PARSETUP/RUN_UCON_REGISTER_FUNCS_INIT with following errors:

   Job: "CL_UPG_UCON_REGISTER_FUNCS"

 Log <sum_path>/SUM/abap/log/UCON_REGISTER_FUNCS.<SID>:

4 ETG011 "ACTION: UNREGISTER_FUNCS"

2EETG012 "Error during unregistering func" "CRR_OPERATION"
2EETG012 "Error during unregistering func" "SUBST_UPG_SGEN_GENER8_RFC"
2EETG012 "Error during unregistering func" "CRR_TRANSFER_STATUS"
2EETG012 "Error during unregistering func" "UPG_RFC_REPLICATOR_WRITE"
2EETG012 "Error during unregistering func" "UPG_GET_DBINFO"
2EETG012 "Error during unregistering func" "SUBST_SLANA_STORE"
2EETG012 "Error during unregistering func" "GET_ICNV_FLAG_CLUSTERS"
2EETG012 "Error during unregistering func" "SUGM_AVMOVES_HANDLING"
2EETG012 "Error during unregistering func" "SUGM_EVAL_LANG_PACKS"

SAP ERP, SAP SOLUTION MANAGER, SAP NETWEAVER, SAP S/4HANA

RUN_UCON_REGISTER_FUNCS_INIT, ucon/rfc/active, ucon, CA, RZ11, PREP_PARSETUP/RUN_UCON_REGISTER_FUNCS_INIT, MAIN_POSTCLEAN/RUN_UCON_REGISTER_FUNCS_FINISH, SAP FIORI FOR SAP S/4HANA, EHP7, EHP8 , KBA , BC-UPG-TLS-TLA , Upgrade tools for ABAP , BC-MID-UCO , Unified connectivity tooling , Problem