Have been using Piwik for a while now with no problems. Today upgraded to 1.5, used automatic upgrade which went fine. Then it redirected back to home page index.php. The script then alternated by either giving a HTTP error 324 (empty response) or Piwik would load with:
"Unable to start session"
Fatal error: Exception thrown without a stack frame in Unknown on line 0
I then tried a manual fresh install and it gave the same error. Running PHP 5.3.2 with config:
'./configure' '--with-pear' '--enable-zip' '--with-apxs2=/usr/sbin/apxs' '--with-openssl' '--with-zlib' '--enable-bcmath' '--with-curl' '--enable-ftp' '--enable-mbstring' '--with-mcrypt' '--with-mysql' '--enable-sockets' '--with-mysqli=mysqlnd' '--with-gd' '--with-ttf' '--with-enable-gd-native-ttf' '--with-freetype-dir'
Running apache 2.2.3
Let me know if you need any more information.
To clarify, other scripts running on the same account/server are working exactly as normal and memory usage etc is normal.
Didn't realise that this had already been posted on  I don't know why the ticket was closed -- this is a bug that obviously more than one user of the software is having.