Previously Primary CPU has fatal error upon switch over, red. PM861A

Hello everyone,

I am facing this issue with a controller in a setup of redundant PM861As. When one particular controller is running as Primary CPU, the redundancy works fine ie. the DUAL LED is lit on both and PRIM is lit on primary. When I initiate switch over, either by pressing INIT, removing power or switching online in Control Builder, the previously Backup CPU correctly becomes the Primary CPU instantly. But the previously Primary now Backup CPU fails to start up due to a strange LDB Overflow error, and I am at loss as to understand why. The message I receive is this:

I 1979-12-31 00:00:00.864 Backup PM stopped by ErrorHandler. Message ||CON_4028_RCU:BkpRCUErrorLDBOverflow, with severity 5

E 1979-12-31 00:00:00.866 FATAL ERROR in d:/br/800xA_CIT/Products/CIT51FP1/BL/5.1.1-2/omega/source/Utilities/ErrorHandle
r/SysAlEv/RedSysAlEvHandlerSIL2.cpp[174]: 873463808 (0x34100000)

I have searched this error in both SolutionsBank and in the manual, and have learnt the following:

From System 800xA Control 5.1 AC800M Configuration:

This event is issued from the RCU Driver if the Backup CPU has been halted due to
an overload situation in the redundancy control HW.
System Simple Event
SrcNameSuffix = _SWTargets;
Message = “(4028) RCU LDB overflow has occured in Backup/trainee
PM";
Severity Level = High;

From SolutionsBank:

FIELD ALERT (doc. no. 3BSE029566D5041): http://abblibrary.abb.com/global/scot/scot296.nsf/veritydisplay/dcaea33052e4b08
dc1257b80005cbc31/$file/3BSE029566D5041_A_en_PM866_backup_CPU_error__LDB_Overfl
ow_in_Backup_.pdf

The controller in question functions correctly in Standalone mode and when it is Primary in a redundant configuration.

The setup is as follows: redundant PM861A, no CEX modules nor ModuleBus Clusters. Only two controllers. The firmwares on the controllers are identical, and the IPs have been correctly configured. Attached is the log for the faulty Primary, now Backup.

Appreciate your answers.