If you have the tabcmd error code 16, Invalid username or password, it’s likely a special character in the password causes the error. For example Tableau interprets %1 as a parameter. In the command line converted it to %%1, with the preceding % acting as an escape character.
Another special character Tableau can’t interpret is # (the hash symbol or pound sign for any Americans). I was unable to find an escape character for that. Therefore I had to change the password, removing the #, to make tabcmd work.
If anyone knows better ways around this than changing the password please write in the comments.