To reproduce:

1. Go to Simple English Wikipedia.
2. Make sure that the Content Translation beta feature is disabled.
3. Go to https://simple.wikipedia.org/wiki/Special:ContentTranslation . Observed: "No such special page", as expected.
4. Go to https://simple.wikipedia.org/wiki/Special:ContentTranslation?campaign=contributions-page . Observed:

Permission error
You do not have permission to edit this page, for the following reason:
The action you have requested is limited to users in one of the groups: $1.

I'd expect the results in 3 and 4 to be the same, and if the error from 4 really has to be displayed, then it should include the groups - but that's probably irrelevant in any case, because it's about preferences and not groups.