This solution applies to Barracuda SSL VPN units running firmware version 2.1 or higher.
Firmware version 2.1 added an option to the PIN configuration to not allow PINs that consist only of sequential numbers. The option can be found at Access Control > Security Settings > PIN, under Allow Sequential Numbers.
By default this should be set to 'Yes', but when set to 'No', any attempts to create PINs using sequential numbers will be blocked.
Link to this page: