Worse still, it appeared only to be broken in Opera... I feared the worst. My first move was to reset all my Gmail settings to default, no luck After that, i left the problem for a few days, hoping it would be quietly fixed, or people would start to complain. Not to be...
I tried a clean install of Opera. Finally! Success So I began to think what could be so different in my version of Opera... After some testing, it boiled down to this: I had set a user agent sub-string, and Gmail was playing nasty.
To clear this setting, visit Opera:Config and search for "id". Reset to default and save.
I haven't reported this to google... I couldn't find an easy way to do so, at least on first glance, but i hope this helps anyone else suffering this problem