Cannot activate newsletter plugin after upgrade

I recently upgraded our site to the latest Debian Testing release – this installs WordPress 3.4.2 and Php 5.4.

It has been a complete nightmare and after finally solving permission issues I go to upgrading Mailpress.

I upgraded Mailpress from vers 3 RC1 to vers 3.

I can activate the mailpress plugin but not the newsletter addon. Activating results in a apache2 500 internal error. I have deactivated all other addons in attempts to isolate the problem with no joy.

I have made the change as per the following post with no effect:
https://www.ads-software.com/support/topic/plugin-mailpress-will-break-site-if-using-php-54?replies=1

I’m fairly desperate. Any ideas?

https://www.ads-software.com/extend/plugins/mailpress/