I have an error when trying to validate change set with a Lightning Community Partner Portal (Partner Central).
Component of type Site.com fails with the error:
In field: siteAdmin - no User named XXX found
I haven't found the setting with a Site Admin field, but I fixed the error by adjusting my usernames in the source and target orgs to follow the convention - appending instance name to the production username.
E.g.: Dev Org (source): email@example.com => Production Org (target): firstname.lastname@example.org
The problem was that my users were created manually in the source and the target orgs and usernames did not follow that agreement.