So, you said you don't have access to the logs? Does your hosting provider have them and isn't letting them go? They should be able to provide you with your website specific error logs, especially if you tell them a time. Like say you activate the plugin, try to access the parts of the theme you need to, and then deactivate the plugin. I'd be interested to see what they said.
I'm leaning towards the theme not cooperating with what Wordpress wants to do and something ought to show in the logs. The confusing part is, of course, whats different between the work and non-working sites.
What theme is it by the way?
tim