Can't Rebuild Permissions

Did a quick update to Organic Groups (which let to an update of Entity Reference) and now my entire site is trashed.

It's crying that it needs to rebuild permissions, but doing so results in

"An AJAX HTTP error occurred. HTTP Result Code: 500 Debugging information follows. Path: /batch?id=26&op=do StatusText: Internal Server Error ResponseText: "

Note that there's nothing after the ":" to help narrow things down

Found a few references to it, and one of the common suggestions was to turn off modules until you found what was causing the error. Did that. Unfortunately what's causing the error appears to be Organic Groups and we absolutely *need* that to run the site, no two ways about it. All the work spent building out the navigation and permissioning structure will be lost without it.

So how do I go about trying to fix this one? I tried rolling back the two modules, but it seems that once the permissions are trashed you're kind of stuck.