Unable to save Billing document

Hi Gurus,

I am trying to create Billing document through VF01 with reference to delivery number after trying to save the document system has taken over to SAP logon screen and Throwed error message, and unable to save the Invoice number. This is happening for two comapany codes only,. I am assuming this is because of number range problem. Please let me know any root cause for this issue.

Regards,

Hari Challa.