Can't recover from No space left on device (3 replies)

While importing old Apache logs in to Piwik using the import_logs.py script, our database server ran out of space.
We resolved this, and we now have 50GB allocated to MySQL and all database tables pass a mysqlcheck.

However we cannot log in to Pwiki anymore. It says:

Warning: session_write_close(): write failed: No space left on device (28) in /web/weblogs2.tcd.ie/libs/Zend/Session.php on line 691
Backtrace -->

#0 Piwik_ErrorHandler(...) called at [:]
#1 session_write_close(...) called at [/var/www/piwikdev.company.com/libs/Zend/Session.php:691]
#2 Zend_Session::writeClose(...) called at [:]

Here's what I have tried.
Delete the session files from tmp/sessions and tmp/templates_c
Re-used another browser, tried another computer
Set the PHP cookie_domain to the same name as the server.

Any ideas? Why does Piwik hold on to this disk full error, even though it's no longer full ?