Thanks for this Wiki... Please add the following to REPACHK2 phase.
Suggestion 1: This error is also applicable to MAIN_INIT/REPACHK1 phase for the same error.
Suggestion 2: In REPACHK2 phase, only DDIC or SAP* user is allowed to login application but can't make changes to customer objects displayed in REPACHK2 phase error. For this one has to follow steps outlined in KBA Article https://launchpad.support.sap.com/#/notes/2244827/E (SUM or SAPup in an execution phase (e.g. PARCONV_UPG, XPRAS_AIMMRG) error: Upgrade still running: Logon not possible)
apologize for the delayed response.
Thanks for your recommendations.
Regarding Suggestion 1: I have included reference to REPACHK1 to the page.
Regarding Suggestion 2: In REPACHK2 the system is not locked against logon, normal users should be able to logon and execute the needed actions.