Undefined Index: instance_settings in field_group_field_group_format_settings()

Yes I received this message too for all of my fieldsets.

Here's the fix (I think). Click on the gear icon under the column Operations. There's a new field that's been added with the most recent release: Extra CSS classes. Just click "Update". Once the blue spinner stops, scroll to the bottom of the page and click "Save". The warning(s) should go away.

My guess is that this will only affect people upgrading from a previous version who already have fieldsets.

Let me know if this solves the problem.

Do you guys still have this after cache clear? This is something that only should happen due to the new property "classes" that was added to fieldsets and divs. On fresh installs it should not happen.
Can you confirm this as truth?

@Stalski I didn't try clearing the cache but the error went away right after I resaved the fields.

Looks like sharikov's problem was fixed too by resaving. I think that counts as a confirmed truth in this case.

Thanks for all the great work on the module. It is a major upgrade from Drupal 6 field groups.

thx ;)
and you are very welcome

...it happens also on fresh install, D7rc2 :( To deal with the error, each time I create new fieldset, I have to click the gear and resave

#6 happens for me as well.

Suppose issue lays in defaults for formatter. I get this error with current core-HEAD, maybe related #986992: Insane etid / {field_config_entity_type} abstraction

This should fix it: http://drupal.org/user/322618
Update from the 1 branch and it should be fixed. The default formatter's instance settings were not declared for the form