Yeah am having this same problem. Had a rather old version of W3 Cache (0.9.3 maybe) as well as other plugins. Did a bunch of updates together and W3 Cache completely killed the website (every query results in 500 error). Removing W3TC plugin files fixes the issue. Tried 0.9.5 and 0.9.5.1, both fail. Reverted back to 0.9.4 for the meantime which is working. If you can’t reproduce issue, send me an email/reply here and I’ll gather more information.
