To be clear (and maybe pedantic), the issue isn't that
X-Ham-Reportis an invalid header name (it's not, non-standard
X-headers are not only extremely common but permitted), but that the header has an invalid value. This is something that could (should) be fixed in SpamAssassin.
If this bug from SpamAssassin then where we can report this issue? Please can you report this bug to SpamAssassin developer?