This doesn't look to me like a bug in nfs-kernel-server, but some wrong output on the debconf pipe that has put things in a wrong state. Reassigning to debconf, though I suspect this is actually a bug in something earlier in the mythbuntu installer.
The error in the logs is:
Creating config file /etc/default/nfs-common with new version
Adding system user `statd' (UID 105) ...
Adding new user `statd' (UID 105) with group `nogroup' ...
Not creating home directory `/var/lib/nfs'.
statd start/running, process 31958
gssd stop/pre-start, process 31991
idmapd start/running, process 32098
Setting up nfs-kernel-server (1:1.2.5-3ubuntu3) ...
Use of uninitialized value $_[1] in join or string at /usr/share/perl5/Debconf/DbDriver/Stack.pm line 111, <GEN0> line 13.
Use of uninitialized value $val in substitution (s///) at /usr/share/perl5/Debconf/Format/822.pm line 83, <GEN1> line 9.
Use of uninitialized value $val in concatenation (.) or string at /usr/share/perl5/Debconf/Format/822.pm line 84, <GEN1> line 9.
Use of uninitialized value $reply in scalar chomp at /usr/share/perl5/Debconf/FrontEnd/Passthrough.pm line 66.
Use of uninitialized value $reply in concatenation (.) or string at /usr/share/perl5/Debconf/FrontEnd/Passthrough.pm line 67.
Use of uninitialized value $reply in split at /usr/share/perl5/Debconf/FrontEnd/Passthrough.pm line 68.
Use of uninitialized value $reply in scalar chomp at /usr/share/perl5/Debconf/FrontEnd/Passthrough.pm line 66.
Use of uninitialized value $reply in concatenation (.) or string at /usr/share/perl5/Debconf/FrontEnd/Passthrough.pm line 67.
Use of uninitialized value $reply in split at /usr/share/perl5/Debconf/FrontEnd/Passthrough.pm line 68.
Use of uninitialized value $reply in scalar chomp at /usr/share/perl5/Debconf/FrontEnd/Passthrough.pm line 66.
Use of uninitialized value $reply in concatenation (.) or string at /usr/share/perl5/Debconf/FrontEnd/Passthrough.pm line 67.
Use of uninitialized value $reply in split at /usr/share/perl5/Debconf/FrontEnd/Passthrough.pm line 68.
Creating config file /etc/default/nfs-kernel-server with new version
dpkg: error processing nfs-kernel-server (--configure):
subprocess installed post-installation script returned error exit status 128
This doesn't look to me like a bug in nfs-kernel-server, but some wrong output on the debconf pipe that has put things in a wrong state. Reassigning to debconf, though I suspect this is actually a bug in something earlier in the mythbuntu installer.
The error in the logs is:
Creating config file /etc/default/ nfs-common with new version perl5/Debconf/ DbDriver/ Stack.pm line 111, <GEN0> line 13. perl5/Debconf/ Format/ 822.pm line 83, <GEN1> line 9. perl5/Debconf/ Format/ 822.pm line 84, <GEN1> line 9. perl5/Debconf/ FrontEnd/ Passthrough. pm line 66. perl5/Debconf/ FrontEnd/ Passthrough. pm line 67. perl5/Debconf/ FrontEnd/ Passthrough. pm line 68. perl5/Debconf/ FrontEnd/ Passthrough. pm line 66. perl5/Debconf/ FrontEnd/ Passthrough. pm line 67. perl5/Debconf/ FrontEnd/ Passthrough. pm line 68. perl5/Debconf/ FrontEnd/ Passthrough. pm line 66. perl5/Debconf/ FrontEnd/ Passthrough. pm line 67. perl5/Debconf/ FrontEnd/ Passthrough. pm line 68.
Adding system user `statd' (UID 105) ...
Adding new user `statd' (UID 105) with group `nogroup' ...
Not creating home directory `/var/lib/nfs'.
statd start/running, process 31958
gssd stop/pre-start, process 31991
idmapd start/running, process 32098
Setting up nfs-kernel-server (1:1.2.5-3ubuntu3) ...
Use of uninitialized value $_[1] in join or string at /usr/share/
Use of uninitialized value $val in substitution (s///) at /usr/share/
Use of uninitialized value $val in concatenation (.) or string at /usr/share/
Use of uninitialized value $reply in scalar chomp at /usr/share/
Use of uninitialized value $reply in concatenation (.) or string at /usr/share/
Use of uninitialized value $reply in split at /usr/share/
Use of uninitialized value $reply in scalar chomp at /usr/share/
Use of uninitialized value $reply in concatenation (.) or string at /usr/share/
Use of uninitialized value $reply in split at /usr/share/
Use of uninitialized value $reply in scalar chomp at /usr/share/
Use of uninitialized value $reply in concatenation (.) or string at /usr/share/
Use of uninitialized value $reply in split at /usr/share/
Creating config file /etc/default/ nfs-kernel- server with new version
dpkg: error processing nfs-kernel-server (--configure):
subprocess installed post-installation script returned error exit status 128