Cannot request a new diff from a derived series for a new upstream version

Looking at the model code, this shouldn't be possible: that error message is produced only if a diff has already been requested between those exact source-package releases. I could imagine the browser code getting releases mixed up though if a package was updated between the time you loaded the overview page and the time you requested the review.

For reference, could you complete the picture of which version was where? AIUI you're requesting two diffs:

Distro / Release / Version → Distro / Release / Version
Debian / ??? / 1.2-9 → Ubuntu / Oneiric / 1.2-9ubuntu1
Debian (?) / ??? / 1.2-9 → Ubuntu (?) / Sid (!?) / 1.2-10

Also, if you reload the UI pages now and try again, is the problem still there?

Distro / Release / Version → Distro / Release / Version
Debian / Sid/ 1.2-9 → Ubuntu / Oneiric / 1.2-9ubuntu1
Debian / Sid / 1.2-9 → Debian / Sid / 1.2-10

Here's the URL I'm using:
https://launchpad.net/ubuntu/oneiric/+localpackagediffs?field.name_filter=&field.package_type=non-ignored&field.package_type-empty-marker=1&field.packageset=84&field.packageset=78

The package in question is tuxguitar.

Also of note, the Precise diffs for tuxguitar are fine.

Sorry, I think the diffs for Precise were the ones I created before. However, I'm experiencing this on the z88 package as well:
        13.0.0+dfsg2-2 to Precise version: 13.0.0+dfsg2-2ubuntu1
        13.0.0+dfsg2-2 to Wheezy version: 13.0.0+dfsg2-3

diff from 13.0.0+dfsg2-2 (in Debian) to 13.0.0+dfsg2-2ubuntu1 was already requested by Micah Gersten