Attempting to activate Shopp fails, and displays a blank page. Blank pages with WordPress typically represent fatal errors in PHP. The circumstances to cause issues such as this will most often occur as a result of the web hosting environment (what technologies are installed on the server) and configuration (how those technologies are configured).
As an example, one incident of this was found to occur in a web hosting installation where Zend Optimizer was enabled. Activating Shopp in this environment caused a segmentation fault in Zend Optimizer which triggered a fatal error in PHP (hidden by WordPress’s error configuration resulting in the blank page) and finally caused the Apache web server to return an HTTP error status 500 (Internal Server Error) with the blank page.
In this case, Shopp was able to be activated by temporarily disabling Zend Optimizer to activate. Re-enabling Zend Optimizer after activation did not cause any further functional issues with WordPress+Shopp.
It should be noted that the above is only indicative of possible causes and that actual causes of failures such as this may be due to similarly extreme and rare environments/configurations.