UPDATE : I have purged the package "nfs-kernel-server" and I have reinstalled it from scratch. it worked for some time and then...it gave the same error as before. so,this is a bug for sure.
** Package changed: ubuntu => nfs-utils (Ubuntu) -- You received this bug notification because you are a member of Ubuntu Bugs, which is subscribed to Ubuntu. https://bugs.launchpad.net/bugs/1884357 Title: nfs-kernel-server does not start because of dependency failure To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/nfs-utils/+bug/1884357/+subscriptions -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs