I agree with Howard (see below) that this may be related to MDL-15284. You should probably add a comment to that issue, rather than creating a new one.
It is very easy to create an issue in the tracker. Go to http://tracker.moodle.org, sign up for a new account (unfortunately it is a separate account), then click the "Create new issue" link in the header. The tracker will take you step-by-step through the issue creation. The details you supplied in your original post are very clear. You could pretty much copy and paste them.
Backup and restore
Import courses from 1.8.2+ to 1.9.1 - Role mapping issue
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