Can't recover from No space left on device

While importing old Apache logs in to Piwik using the 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/ on line 691
Backtrace -->

#0 Piwik_ErrorHandler(…) called at [:]
#1 session_write_close(…) called at [/var/www/]
#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 ?

can you run $df on the server to see if no partition is full?

Of course it’s not full. None of the paritions are full. There is 25 GB free.
It was full, but we fixed that. But piwik is holding on to this error.

Found the problem - the session save path was incorrect. Once I fixed that it was happy.