Upgrade from 5.7.20 to 5.7.21 fails on pre-installation script

Hello,

I’m having trouble to upgrade an installation of 5.7.20 to 5.7.21 on a Ubuntu 16.04 system. Upgrade fails with pre-installation script error but I wasn’t able to debug it. After failing to upgrade system asks to execute “apt-get install -f” but this fails also.

root@ais12-us:~# apt-get upgrade
Reading package lists... Done
Building dependency tree 
Reading state information... Done
You might want to run 'apt-get -f install' to correct these.
The following packages have unmet dependencies:
percona-server-server-5.7 : PreDepends: percona-server-common-5.7 (= 5.7.20-19-1.xenial)
Depends: percona-server-client-5.7 (= 5.7.20-19-1.xenial) but 5.7.21-20-1.xenial is installed
E: Unmet dependencies. Try using -f.
root@ais12-us:~# apt-get -f install
Reading package lists... Done
Building dependency tree 
Reading state information... Done
Correcting dependencies... Done
The following additional packages will be installed:
percona-server-server-5.7
The following packages will be upgraded:
percona-server-server-5.7
1 upgraded, 0 newly installed, 0 to remove and 31 not upgraded.
1 not fully installed or removed.
Need to get 0 B/25.2 MB of archives.
After this operation, 575 kB of additional disk space will be used.
Do you want to continue? [Y/n] y
Reading changelogs... Done
Preconfiguring packages ...
dpkg: warning: files list file for package 'percona-server-server-5.7' missing; assuming package has no files currently installed
(Reading database ... 174221 files and directories currently installed.)
Preparing to unpack .../percona-server-server-5.7_5.7.21-20-1.xenial_amd64.deb ...
................
dpkg: error processing archive /var/cache/apt/archives/percona-server-server-5.7_5.7.21-20-1.xenial_amd64.deb (--unpack):
subprocess new pre-installation script returned error exit status 1
Errors were encountered while processing:
/var/cache/apt/archives/percona-server-server-5.7_5.7.21-20-1.xenial_amd64.deb
E: Sub-process /usr/bin/dpkg returned an error code (1)

Any hints on where to look or what to try?

Thanks in advance.