I can't remember which version they fixed this in, but I recall there being a change that allowed you to unset the read-only flag. Older versions (and again, I'm not sure where the cutoff is) did not allow you to unset read-only once set, resulting in an error message like the one you're showing.
Have you tried an outright:
PUT /indexname/_settings
{
"index.blocks.read_only": false
}
...to set the index back to full read/write capacity?