Although I do not know the answer to the question presented, one possibility may be due to the fact that the directory 'myADusername' is included in the share path. I just now tested my ability to log in to our server, and it worked fine as long as I did not include a directory in the share path:
This will prompt you for a password (I was not able to successfully include a password in the command).
smbclient \\\\server.domain.tld\\share -U 'domain\myADusername'
Including a directory in the share path
returns the following message:
smbclient \\\\server.domain.tld\\share\directory -U 'domain\myADusername'
Enter XXXX\user's password:
Domain=[XXXX] OS=[Windows X.X] Server=[Windows X LAN Manager]
tree connect failed: NT_STATUS_BAD_NETWORK_NAME