Unexpected response from WordPress" when activating Snapshot

When activating Snapshot Pro on the WPMU dashboard, I get this error at the top of the page: "Unexpected response from WordPress".
How do we diagnose this to determine what the query was, what the response was, and what can be done to address the issue?

I have full WP debugging/logging on but nothing is logged.
I thought this was permissions for this new installation, but after opening the site permissions up (internal server) the error persists.
Then I guessed it was a PHP error, and yes, PHP was reporting over 256M memory usage on this unused server (no clue about that yet) while the WP Query plugin reports only about 8M. I increased php.ini to 300M temporarily and that eliminated the PHP error, but not this activation error.

I can poke around in code but I think this is an opportunity to make the dashboard a little smarter about reporting issues, or at least to document for posterity here how we should approach this.