There seems to be some sort of error happening when the permissions try to extend from FB. I see this panels.accounts.extending script try to run and fails.
Solved after many frustrating attempts. Needed to delete connection, clear cache and cookies for FB / make / IG then reconnect via the IG module FIRST. This error seems to come if the FB module was connected first - then the IG one would not extend authorization. Seems backwards!
I ran into the same issue, and you solution works perfectly! Deleted the FB authorization and was able to finally add the IG connection and get the FB connection back again as well. Thanks!