Besides this error I have doubts that I can create my own certificate. It sounds like we need to buy a certificate from VeriSign or another certificate authority.
This is correct, you will need to purchase a code signing certificate from one of the companies that are selling this kind of certificates.
How do we change the publisher unknown to the name of our company?
This can be done only by signing the package with a valid certificate.
Can we do this with a certificate which I create? If yes, do you know what causes the error 0x80880253?
No, you will need to purchase the certificate, you cannot use a certificate created by you. The error code is returned by the sign tool used, not
by Advanced Installer. This probably happens because this is not a valid certificate.
If no, is there a way to get a certificate for free which will satisfy our requirements of listing our company as the publisher.
From what I know there is no company that can offer you code signing certificates for free. Maybe there are companies with smaller prices, but I am not aware
of any company which can offer it for free. You can try a search on Google, maybe you will find something.
Please let us know if you have other questions.