Hello. In my setup (amd64), this package FTBFS... 100% of the time on machines with 1 CPU 50% of the time on machines with 2 CPUs
So, it does not seem to be architecture-specific (s390x). I suggest trying GRUB_CMDLINE_LINUX="nr_cpus=1" since that's what apparently makes the failure rate to be higher. If that does not work, please contact me privately and I will gladly provide a VM to debug. Thanks.
