Some people receive Unexpected Sign In failure error when try to sync Lightroom to Lightroom mobile. Typically the error message from Lightroom Desktop was “Lightroom has experienced an unexpected Sign In failure. Please try again later. (attempt to index a nil value).” You many also receive a server not responding message.
This is a known issue, with following workaround will allow you sign-in access by enable sharing in the System Preferences.
1. Open System Preferences in Mac OS.
2. Select Sharing.
3. Enter your computer name into the Computer Name field. The computer name you need to enter into this field is in the text directly under this field, and contains a .local suffix. The computer name you need to enter is this name, but without the .local.
Note: If the computer name in the text is just .local, enter whatever computer name you wish into the Computer Name field, but do not include the .local.
4. Close System Preferences.
More detail at Adobe Blog [Unable to Login to Lightroom mobile – Unexpected Sign in Failure – Solution]