I'm also receiving this error, I have a developer account, and previously the only way I could use Sideloadly with my dev account was with Anisette disabled and an app-specific password. Now nothing I try allows me to use my dev account with Sideloadly. My specific error message is:
ERROR: Guru Meditation f8880e@876:3aea77 Failed: (3019) Authentication Error. Xcode 7.3 or later is required to continue developing with your Apple ID.
When I try with Anisette enabled, and my normal password, I get the following error:
Got error: Login failed (-36607): Unable to sign you in to your Apple ID. Try again later.
Retrying with alt Anisette
ERROR: Guru Meditation f8880e@244:3aea77 Login failed (-36607): Unable to sign you in to your Apple ID. Try again later.
And with Anisette enabled, and my app specific password, I get the following, which I'm sure is expected:
ERROR: Guru Meditation f8880e@244:3aea77 Login failed (-22406): Your Apple ID or password is incorrect.
Any help would be appreciated, thanks!