So after months of painstaking development, debugging and optimization, it comes to this:
Plugin could not be activated because it triggered a fatal error.
Despite feeling the urge to smooth this over with a clever and technically impressive explanation, I need to call it what it is: embarrassing. I simply forgot to test plugin activation. To reduce memory usage, I took out a line of code that was not needed for the plugin to run – but is needed for the plugin to activate.
My apologies to anyone who was discouraged by this. Thanks to rconsidine for the support forum report, which led to an easy fix.
Role Scoper 1.1 RC2 also fixes a problem with the handling or restrictions when using the Custom Taxonomies plugin with one or more hierarchical custom taxonomies.