If "Modified domain cookie(s)" violation is detected, then this means that this cookie (or wildcard, which matches this cookie) is defined as enforced on "Security ›› Application Security : Headers : Cookies List" page.
"Enforced" means that cookie can not be modified by user and in case of any modification "Modified domain cookie(s)" violation must be detected.
"Enforced" need to be used for cookies (like session ID), which are set by application via "Set-Cookie" header and can not be modified by user.
If in your case you expect, that cookie can be modified, then you need to change it's type to "Allow".
Thanks, Ivan