A 1046 rejection will be caused by invalid filing credentials being used at the point of submission or that the UTR number is wrong. If you are filing for a partnership then you will need to check the UTR number for the partnership, along with the associated individual partners.
 

If you are filing for a CT600 return then you will also need to check the three digit tax office number and the start and end of the accounting period.
 

The password needs to be between 8 and 12 characters and not include any unusual characters. HMRC do allow you, on their website, to create a password which exceeds 12 characters and include unusual characters but this then causes their server to return the 1046 rejection.