Hi,
Nexus 7000: Clear text passwords cannot contain dollar signs ($) or spaces anywhere in the password. Also, they cannot include these special characters at the beginning of the password: quotation marks (" or '), vertical bars (|), or right angle brackets (>).
Nexus 5000: Clear text passwords can contain alphanumeric characters only. Special characters, such as the dollar sign ($) or the percent sign (%), are not allowed.
Not sure is the @ sign is between those special characters, but may be the case.
Hope this helps.