problem with pg_upgradecluster from postgresql 8.1 to 9.1

Hello,
I know this is a quite old system, but I hope someone will give some
hint on how to find the problem.

I am trying to upgrade an old 8.1 cluster to 9.1 on an ubuntu dapper
system. 9.1 is the latest installable form PGDG on that old dapper, in
fact packages have been installed without problems.

When running pg_upgradecluster, the procedure stop just after new
cluster creation. No errors are available on the logs: the new cluster
seems working, but the upgrade stop and write "Could not start target
cluster".

Here are all details

1. the original system have two clusters. Only one is being upgraded:

root(at)atf-810:/var/lib/postgresql/8.1# pg_lsclusters 
Version Cluster   Port Status Owner    Data directory                     Log file
8.1     main      5432 online postgres /var/lib/postgresql/8.1/main       /var/log/postgresql/postgresql-8.1-main.log
8.1     q3        5433 online postgres /var/lib/postgresql/8.1/q3         /var/log/postgresql/postgresql-8.1-q3.log

2. this is the procedure that fails

root(at)atf-810:/var/lib/postgresql/8.1# pg_upgradecluster 8.1 main
Stopping old cluster...
Disabling connections to the old cluster during upgrade...
Restarting old cluster with restricted connections...
Creating new cluster (configuration: /etc/postgresql/9.1/main, data: /var/lib/postgresql/9.1/main)...
Moving configuration file /var/lib/postgresql/9.1/main/postgresql.conf to /etc/postgresql/9.1/main...
Moving configuration file /var/lib/postgresql/9.1/main/pg_hba.conf to /etc/postgresql/9.1/main...
Moving configuration file /var/lib/postgresql/9.1/main/pg_ident.conf to /etc/postgresql/9.1/main...
Configuring postgresql.conf to use port 5434...
Disabling connections to the new cluster during upgrade...
The PostgreSQL server failed to start. Please check the log output:
2017-08-17 14:05:18 CEST LOG: database system was shut down at 2017-08-17 14:05:17 CEST
2017-08-17 14:05:18 CEST LOG: autovacuum launcher started
2017-08-17 14:05:18 CEST LOG: database system is ready to accept connections
2017-08-17 14:05:18 CEST LOG: incomplete startup packet
Error: Could not start target cluster

3. the new cluster is correctly running

root(at)atf-810:/var/lib/postgresql/8.1# pg_lsclusters
Version Cluster Port Status Owner Data directory Log file
8.1 main 5432 online postgres /var/lib/postgresql/8.1/main /var/log/postgresql/postgresql-8.1-main.log
8.1 q3 5433 online postgres /var/lib/postgresql/8.1/q3 /var/log/postgresql/postgresql-8.1-q3.log
9.1 main 5434 online postgres /var/lib/postgresql/9.1/main /var/log/postgresql/postgresql-9.1-main.log

4. the log of new cluster is exactly what alredy printed by
pg_upgradecluster. The last lines of old cluster log are:

oot(at)atf-810:/var/lib/postgresql/8.1# tail -20 /var/log/postgresql/postgresql-8.1-main.log
LOG: incomplete startup packet
LOG: autovacuum: processing database "postgres"
LOG: incomplete startup packet
LOG: incomplete startup packet
LOG: received smart shutdown request
LOG: shutting down
LOG: database system is shut down
LOG: could not load root certificate file "root.crt": No SSL error reported
DETAIL: Will not verify client certificates.
LOG: database system was shut down at 2017-08-17 14:05:10 CEST
LOG: checkpoint record is at A8/30BC8A4
LOG: redo record is at A8/30BC8A4; undo record is at 0/0; shutdown TRUE
LOG: next transaction ID: 3262147080; next OID: 206065
LOG: next MultiXactId: 3; next MultiXactOffset: 5
LOG: database system is ready
LOG: incomplete startup packet
LOG: transaction ID wrap limit is 299003, limited by database "postgres"
LOG: autovacuum: processing database "postgres"
LOG: incomplete startup packet
LOG: autovacuum: processing database "template1"

5. this is the version of postgresql-common, that includes
pg_upgradecluster command:

root(at)atf-810:/var/lib/postgresql/8.1# apt-cache policy postgresql-common
postgresql-common:
Installed: 137.pgdg50+1~1353587801.bzr1228.24
Candidate: 137.pgdg50+1~1353587801.bzr1228.24
Version table:
*** 137.pgdg50+1~1353587801.bzr1228.24 0
500 http://apt.postgresql.org lenny-pgdg/main Packages
100 /var/lib/dpkg/status
53ubuntu3 0
500 http://old-releases.ubuntu.com dapper/main Packages

Thank you,
Giuseppe