Allen Bradly, PLC5 Hot backup. Problems getting online with redundant PLC

Hello all.

I've got an issue with an Allen Bradly PLC5/80c.
We've got a hot backup configuration in which our PLC1 is usually the active PLC.
In the current issue I opened the program of PLC2 and tried to get online ('Go Online').

Normally the status changes to 'ONLINE', but now I received a pop-up as given in attachement 'Melding bij online gaan - PLC2'.
I thought I recognised this as a difference between on- and offline program. I used 'browse' to select the program and after that I selected the third (greyed-out) option 'Upload logic, merge ....'. In my experience this should solve the issue, but not this time. After the program-structure was rebuild the PLC-status went to 'FAULTED', in stead of 'ONLINE'.
Using the arrow next to the status (see attachement 'PLC-status') I went to 'Goto Error' from where I received the pop-up in attachement 'Status pop-up', mentioning a check sum error.

In the field the PLC2 showed a flashing LED 'PROC'. We have tried 'Clear Fault' (see attachement 'PLC-status') and the LED turned off.
When trying to go online again in RSLogix the status changes to 'Remote Prog', not 'ONLINE' as I am used to.

As I am only familiair with changing the logic code and not so much with settings and all I do not know how to get back to what I am used to: Going online and therewith get to the status 'ONLINE'.
Anybody got some tips on this?