pg_upgrade and wraparound

Hello!

I've been trying to upgrade a postgresql cluster from 9.6 to 10. I've
executed the pg_upgrade with the following options:

/usr/lib/postgresql/10/bin/pg_upgrade -b /usr/lib/postgresql/9.6/bin/ -B
/usr/lib/postgresql/10/bin/ -d /var/lib/postgresql/9.6/main -D
/var/lib/postgresql/10/main -o ' -c
config_file=/etc/postgresql/9.6/main/postgresql.conf' -O ' -c
config_file=/etc/postgresql/10/main/postgresql.conf'

The upgrade operation failed after several hours with the following error:

database is not accepting commands to avoid wraparound data loss in
database with OID 0

Earlier in the log there are a lot of messages like

pg_restore: executing BLOB 1740736966
pg_restore: WARNING: database with OID 0 must be vacuumed within 1000279
transactions
HINT: To avoid a database shutdown, execute a database-wide VACUUM in that
database.
You might also need to commit or roll back old prepared transactions.
pg_restore: WARNING: database with OID 0 must be vacuumed within 1000278
transactions
HINT: To avoid a database shutdown, execute a database-wide VACUUM in that
database.
You might also need to commit or roll back old prepared transactions.

I've tried to do VACUUM FULL on my 9.6 cluster on all databases and then
retried the pg_upgrade - it failed in the same way.

Also to be noted, earlier this cluster was succesfully upgraded with
pg_upgrade using similar parameters from older versions (at least 2 times,
something like 9.1 -> 9.3, 9.3 -> 9.6). The database is around 700 GB and
has very many pg_largeobjects in it.

What could be the reason of this and how can I perform my upgrade?

Thanks in advance,
Alexander