Maybe a possible fix? I tried doing as linegel did but there was no button to accept newer terms for me. I restarted and tried again to no avail. Afterwards I reinstalled sideloadly and tried disabling "use anisette authentication" and tried to login with an app specific password, that didn't work either. I noticed that my device wasn't being detected by sideloadly via wifi so i uninstalled itunes, reinstalled, then connected my device via wire, turn sync over wifi off and on, restarted and sideloadly starting again for ALL 3 ACCOUNTS that i've previously tried to use with sideloadly for it to just give the error. I only tried signing into https://developer.apple.com/account/ with one of the emails which didn't fix anything. But after whatever I did, all 3 previous accounts started to work again.