once committing an interface rename under:

services dns dynamic
rename interface eth0.400 to interface pppoe0

the ddclient cache file becomes unaccesible by the ddclient service although the file does have (RW) permissions.

once renaming the interface it is not possible to delete or run show status of the dynamic dns interface.

the errors that shows up:

delete service dns dynamic 
commit

[ service dns dynamic ]
Traceback (most recent call last):
  File "/usr/libexec/vyos/conf_mode/dynamic_dns.py", line 281, in <module>
    apply(c)
  File "/usr/libexec/vyos/conf_mode/dynamic_dns.py", line 261, in apply
    if os.path.exists(dyndns['cache_file']):
TypeError: 'NoneType' object is not subscriptable
delete [ service dns dynamic ] failed

by renaming the interface back to the original interface and committing the error still occurs.

it seems like there's a permissions issue with the ddclient.cache file.
However:
when setting the permissions to 666 the same issue occurs.
when the file is deleted it's recreated but with 600 permissions.