Leapp fails after the reboot, following a conversion from CentOS

Issue

The leapp-upgrade.log shows the following traceback:

<TIMESTAMP> localhost upgrade[706]: ====> * update_etc_sysconfig_kernel
<TIMESTAMP> localhost upgrade[706]:         Update /etc/sysconfig/kernel file.
<TIMESTAMP> localhost upgrade[1649]: Process Process-246:
<TIMESTAMP> localhost upgrade[1649]: Traceback (most recent call last):
<TIMESTAMP> localhost upgrade[1649]:   File "/usr/lib64/python2.7/multiprocessing/process.py", line 258, in _bootstrap
<TIMESTAMP> localhost upgrade[1649]:     self.run()
<TIMESTAMP> localhost upgrade[1649]:   File "/usr/lib64/python2.7/multiprocessing/process.py", line 114, in run
<TIMESTAMP> localhost upgrade[1649]:     self._target(*self._args, **self._kwargs)
<TIMESTAMP> localhost upgrade[1649]:   File "/usr/lib/python2.7/site-packages/leapp/repository/actor_definition.py", line 74, in _do_run
<TIMESTAMP> localhost upgrade[1649]:     actor_instance.run(*args, **kwargs)
<TIMESTAMP> localhost upgrade[1649]:   File "/usr/lib/python2.7/site-packages/leapp/actors/__init__.py", line 289, in run
<TIMESTAMP> localhost upgrade[1649]:     self.process(*args)
<TIMESTAMP> localhost upgrade[1649]:   File "/usr/share/leapp-repository/repositories/system_upgrade/el7toel8/actors/updateetcsysconfigkernel/actor.py", line 20, in process
<TIMESTAMP> localhost upgrade[1649]:     updateetcsysconfigkernel.update_kernel_config('/etc/sysconfig/kernel')
<TIMESTAMP> localhost upgrade[1649]:   File "/usr/share/leapp-repository/repositories/system_upgrade/el7toel8/actors/updateetcsysconfigkernel/libraries/updateetcsysconfigkernel.py", line
 9, in update_kernel_config
<TIMESTAMP> localhost upgrade[1649]:     path])
<TIMESTAMP> localhost upgrade[1649]:   File "/usr/lib/python2.7/site-packages/leapp/libraries/stdlib/__init__.py", line 192, in run
<TIMESTAMP> localhost upgrade[1649]:     result=result
<TIMESTAMP> localhost upgrade[1649]: CalledProcessError: Command ['/bin/sed', '-i', 's/^DEFAULTKERNEL=kernel$/DEFAULTKERNEL=kernel-core/g', '/etc/sysconfig/kernel'] failed with exit code