The beta doesn't work for me, but I don't think it's Qt5 related.
The error message I get when connecting to Marvin is the same as I got when I uninstalled and reinstalled the plugin a couple of weeks ago. The "solution" then was to copy back the old plugin-files from a backup. That's of course not possible with the beta.
It looks like I have to wait until Philantrop gets some free time. There is a few differences in behaviour after dismissing the error, but I'm not sure how helpful they are since something is obviously wrong at my end. Let me know if you want to know anyway.