I tried a cold reboot, but it did nothing. Also, applying the latest core update did nothing. In the end I had to replace the cards; with the new ones, the firmware update was successful after one or two retries (?) but in the end it worked.
I have to check if I can restore the firmware on the two failed cards by installing them on a un-patched system.
Anyway, I'll open a case with Nortel because it seems like a serious issue.
Regards,
Radu.