I've restarted this thread because the last one got locked through inactivity.
Our computing boffins have finally managed to find the incompatability (though not the underlying cause).
It turns out that there seems to be some interference between the HTML toolbar button and the multilang2 toolbar button in ATTO.
In the ATTO toolbar configuration dialogue, you have to load html before multilang2.
In other words,
- other = fullscreen,preview,multilang2, html
- other = fullscreen,preview,html, multilang2
I note, for what it is worth, that multilingual code problems are often harder to debug because many users have a monolingual Moodle instance and cannot reproduce the error.
I will commuincate this in the Tracker too and contact the multilang2 coders.
Thanks for your interest.
I've replied to your comment on the multilang2 filter plugin support page. While that plugin is not the multilang2 Atto plugin (atto_multilang2) you seem to have problems with, I hope I could give you a hand with this issue.
But first I'd need to know the exact versions of Moodle and atto_multlang2 plugin you are using.