2017-10-23 Update: This problem has just been solved via an upgrade.
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Dear teachers,
please not that the activity "checkmark" does currently not support group mode due to a bug.
Once you set group mode in the settings of any checkmark activity or by clicking on the group mode icon, no student submissions cannot be displayed - you will only get "error reading from database".
I forwarded this bug to the developers of the plugin and will keep you updated about upgrades in this thread. Thanks for your consideration.