Well, I think the problem does have to do with Instance, not Instagram accounts. Reset my password and didn't fix.
I noticed this has been going since the name change from Itsdagram to Itsdagram. After some days Instance appeared blocked in background tasks, and even marking to turn background tasks back the next time it didn't work, and I got a message asking to reinstall itsdagram (when the app was already name instance).
I'd guess the windows phone management of background tasks is some way confused with the old and new names.