I've tested this some more and my problem isn't about the feedback block, or previewing . It's simply about accessing feedbacks on the moodle front page - specifically, feedbacks that were created before upgrading moodle to from 1.9 to 2.2.
If I create a new feedback on the front page, everything is fine. standard authenticated users can access and complete the feedback.
I have several old feedbacks that were on my front page before I upgraded moodle. If I log on as a student, and I click one of the old feedbacks, I get an "invalid course module ID" error.
If I log on as an admin, and I click on the old feedback, I can access and complete it without any problems.
The permissions look OK - authenticated users can complete and view feedbacks. (see the permissions of an example student below).
I don't need my old feedbacks any more, and as long as new ones work I haven't got a real problem. So don't worry about it if this is just an odd local problem.
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Permissions for user student f
Capability Allowed
Activity: Feedback
Complete a feedback
mod/feedback:complete Yes
Create private template
mod/feedback:createprivatetemplate No
Create public template
mod/feedback:createpublictemplate No
Delete completed submissions
mod/feedback:deletesubmissions No
Delete template
mod/feedback:deletetemplate No
Edit items
mod/feedback:edititems No
Map courses to global feedbacks
mod/feedback:mapcourse No
Receive email notification
mod/feedback:receivemail No
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Thanks for the feedback module and your great support for it, Andreas - it's much appreciated.