Can't edit blocks

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All sorts of problems!

First some history. The site was working fine until I upgraded to 5.6.0.1 (with the intermediate upgrades done first). After the upgrade editing content blocks no longer worked. I could go into edit mode and edit the content, but saving the edit resulted in an error. After looking through forum entries, I thought it might be an offending add-on. However, removing the add-on, made the site completely disappear. I could get no pages to appear and therefore couldn't get to the dashboard to do anything. After looking through some other forum posts, I was able to roll back to 5.0.1 and get the site back.
But...
Editing still doesn't work. Adding a content block is OK, but editing one give this error message:
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An unexpected error occurred. mysql error: [1062: Duplicate entry '916' for key 'PRIMARY'] in EXECUTE("INSERT INTO btContentLocal(bID,content) VALUES ('916',' ')") < Back to Home
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Canceling and exiting edit mode seems to log me out. To do any other editing, I need to log in again.
Canceling and trying to edit the block again give an "Access denied" message.

Also, the dashboard is missing components:
Add Functionality has no subcategories.
Settings and System & Maintenance have subcategories, but choosing one gives a blank page.
The other dashboard categories have (it seems) all the subcategories, but not all work.

Any help would be greatly appreciated. (I won't be able to test any of the suggestions for a couple of days, however.)
Thanks.