I know that the locking code in the certificate seems to be causing problems, I do not have the time to check and test all of that code right now (each activity type has its own code), however, I did update the Activity Locking code to make it easier to use, so that might be an option for you (until Moodle 2.0 comes out).
You can get the info here
Custom certificate (plugin)
Locking
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