Incorrect message on wrong Block translation

On a multi-lingual site, I configured a custom block with Full HTML text format and 'make this block translatable' marked. When I hit 'Save and translate', I get a nice warning telling me:

The string blocks:block:8:body for textgroup blocks is not allowed for translation because of its text format.

This doesn't happen when I do the following:

- instead of 'Save and translate', I hit 'Save'
- go to the tab 'Translate' and choose the item I want to translate
- enter the translation and hit 'Save translation'

This returns a message:

Translations saved.

However, the translation is not saved and the status still mentions 'not translated'.