AltServer cannot connect to Apple ID

I'm seeing this error message when I try to install the AltStore.
It may happen because I'm using an Apple ID with a + sign, like johndoe+apple@gmail.com (checked, not relevant)

The error states:

AltStore could not be installed onto iPhone.
AltServer could not sign in with your Apple ID. The data is not in the correct format.

NSCocoaErrorDomain 3840

Debug Description
Encountered unknown tag html on line 1

Error Description
AltServer could not sign in with your Apple ID. The data is not in the correct format.

Failure
AltServer could not sign in with your Apple ID.

Failure Reason
The data is not in the correct format.

Title
AltStore could not be installed onto Andrea's iPhone.

kCFPropertyListOldStyleParsingError
Error Domain=NSCocoaErrorDomain Code=3840 "Malformed data byte group at line 1; invalid hex" UserInfo=(NSDebugDescription=Malformed data byte group at line 1; invalid hex}

Start by reproducing the Apple ID sign-in and AltStore installation flow described in the issue, using the reported NSCocoaErrorDomain 3840 response as the starting symptom. Trace where the malformed data is received and determine what successful sign-in should return. Done means the reported Apple ID can sign in and AltStore can be installed without the format error.