Since webmin version 1.61 I got an error on adding/updating A RR with message "reverse zone not found".
It seems to be a bug because after some investigation I found out that the bind8-lib.pl returns an empty result
in function find_reverse. I do not use DNS views, but the subroutine was called with view "any" instead of empty value.
When I changed the expression '' to 'any' in line 735 it works.
Yes, this is a known bug - however, Webmin 1.620 should fix it.