For the original error it appears to be a one off error, someone goofed or Tim lost his keys. It appears to be that the error occurred after the delete vote was submitted but before the review was completed so probably as it tried to complete the review. Therefore, when you opened the review again (refreshed the page) as you had already actioned it you couldn't do another action on it again.
You can replicate this behaviour by flagging and/or VTDing outside of queue and then opening the link to the review up. I did this over on SFF where I voted to delete and flagged an answer outside of queue and then opened the review up and it gave the same text as you "This item is not reviewable."
On a related note here had you skipped or simply closed the review and opened the link up again you should have been able to do the review. (Unless of course you actioned the post outside of review, it completed before you reopened the link or you did the FP or LA on it).