Pulseri A signature for a group could be reused for a person whose id was the same, for example user 7 and company 7. Signatures are now made over person:<id> for /identify and /track, and over group:<id> for /group. A signature of the bare id is no longer accepted and gets 401 invalid_signature.
Change your server to sign with the prefix. The secret stays the same. Code examples are in the REST API guide and on how signing works.