HHVM memcached extension does not set compression flags properly, see upstream bug https://github.com/facebook/hhvm/issues/8028
php_memcached 3.x expects this flag to be set and fails to decompress values saved with the HHVM extension.
The long-term fix is to get the upstream HHVM patch merged. The short-term fix is to build our own packages with that patch. We will however have the issue of all existing keys in the caches. If expiry time for the longest of those is too long, we should also build our own php_memcached packages for 3.0.x and deploy those until all affected keys are gone.