One of the arguments to the command was an int instead of a string. This didn't cause a problem for run_subprocess() itself but caused loggerinst.debug() to error when it tried to format the log message using the argument list.

Reproducer:
c2r vagrant box created using:

UEFI=1 SYSTEM=ol7 vagrant up
In the vagrant box, installed the standard kernel with yum. Rebooted.
convert2rhel --debug -k [ACTIVATION_KEY] -o [ORGANIZATION]

Traceback when adding the new EFI bootloader

[03/03/2022 14:28:07] DEBUG - ESP device number:

Unknown macro: {'major'}

[03/03/2022 14:28:07] DEBUG - The new UEFI binary: /boot/efi/EFI/redhat/shimx64.efi
Adding 'Red Hat Enterprise Linux 7' UEFI bootloader entry.
[03/03/2022 14:28:07] DEBUG - Traceback (most recent call last):
File "/usr/lib/python2.7/site-packages/convert2rhel/main.py", line 115, in main
post_ponr_conversion()
File "/usr/lib/python2.7/site-packages/convert2rhel/main.py", line 235, in post_ponr_conversion
grub.post_ponr_set_efi_configuration()
File "/usr/lib/python2.7/site-packages/convert2rhel/grub.py", line 570, in post_ponr_set_efi_configuration
_replace_efi_boot_entry()
File "/usr/lib/python2.7/site-packages/convert2rhel/grub.py", line 505, in _replace_efi_boot_entry
efibootinfo_new = _add_rhel_boot_entry(efibootinfo_orig)
File "/usr/lib/python2.7/site-packages/convert2rhel/grub.py", line 415, in _add_rhel_boot_entry
stdout, ecode = utils.run_subprocess(cmd, print_output=False)
File "/usr/lib/python2.7/site-packages/convert2rhel/utils.py", line 135, in run_subprocess
loggerinst.debug("Calling command '%s'" % " ".join(cmd))
TypeError: sequence item 5: expected string or Unicode, int found