2471797 - Unresolved transport requests found in phase XPRAS_AIMMRG or SHADOW_IMPORT_UPG1

Symptom 1:

Upgrade with SUM tool stopped in downtime phase XPRAS_AIMMRG with below error:

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Checks after phase MAIN_NEWBAS/XPRAS_AIMMRG were negative!

Last error code set:
Unresolved transport requests found
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Below error message in XPRASUPG.ELG:

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A2EE/CTSPLUG/CTS_PLG_MSG 052 tp return code "200", RFC message: " "" "" "
2EEPU133 Errors occurred during post-handling "/CTSPLUG/ACT_AFTER_IMP" for "PLGD" "L"
3 EPU135 "/CTSPLUG/ACT_AFTER_IMP" belongs to package "/CTSPLUG/CCTSACTIVATION"
2EEPU136 The errors affect the following components:
2EEPU137 "BC-CTS-TMS" ("Transport Management System")
1 ETP111 exit code : "8"
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Symptom 2:

Or the same error message in the SHADOW_IMPORT_UPG1 phase:

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Checks after phase MAIN_NEWBAS/SHADOW_IMPORT_UPG1 were negative!

Last error code set:
Unresolved transport requests found
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• SAP Netweaver
• SAP ERP

Unresolved transport requests found, XPRAS_AIMMRG, SHADOW_IMPORT_UPG1 , KBA , BC-UPG-TLS-TLA , Upgrade tools for ABAP , BC-CTS-TMS , Transport Management System , Problem