Found this on two of my WordPress installations (one running current stable and one running bleeding-edge WordPress).
After activating, Dashboard would return error 500. Turned on debugging in PHP and found this error repeated around 82 times before running out of memory and dying:
Notice: Trying to get property of non-object in /var/www/vhosts/blahblah/problem-with-post-edit-buttonttpdocs/blog/wp-content/plugins/simple-press/sp-api/sp-api-primitives.php on line 761
I "fixed" this by checking for "if ((isset($spThisUser))&&(isset($spThisUser->tracki)))", but I'm not entirely sure where $spThisUser comes from, so haven't looked into working out why it isn't initialised.
This was a clean installation on these blogs, can't 100% confirm that was the reason for the crash tho.