An HTTP 404 error "Page not found" is received while trying to approve actions via link received in policy email.
If a policy is created under a Flex Org sub-org, the delegated action is only accessible from that sub-org.
Depending on user group assignments, when logging into the platform a user may not be in the Flex Org the policy org was created. This may be why they are not unable to access the desired action page, and why a 404 error is returned.
The user needs to switch to the organization under which the policy and then click the link to access the action page.
If the user is in the correct sub-org and still not able to access the action page, please open a support case.