6.13 to 6.14 upgrade fails on database upgrade

It fails with errors starting

"An error occurred. http://xxxxxxxxx/update.php?id=33&op=do
Warning: Can't connect to local MySQL server through socket '/var/lib/mysql/mysql.sock' (111) query: INSERT INTO watchdog (uid, type, message, variables, severity, link, location, referer, hostname, timestamp) VALUES (2, 'php', '%message in %file on line %line.', 'a:4:{s:6:\"%error\";s:12:\"user warning\";s:8:\"%message\";s:137:\"Lost connection to MySQL server during query\nquery: ALTER TABLE menu_router ADD INDEX tab_root_weight_title (tab_root(64), weight, title)\";s:5:\"%file\";s:56:\"/var/www/html/content/includes/database.mysql-common.inc\";s:5:\"%line\";i:434;}', 3, '', etc. "

I notice that default.settings.php has been changed to add the line

ini_set('session.use_cookies', 1);

I don't know if this makes any difference. And it is not usual to expect to change the settings file.

All upgrades that I have attempted have worked perfectly in the past.

Anyone any ideas?

-----------

Now resolved.

Solution: disable all non-core modules first. [I upgraded another site at the same time, also without disabling all non-core modules. That upgrade worked perfectly, this one did not. I don't know why.]

Thanks,

Martin