Interestingly this setting seems not to be used in this case, as it contains redirectPort="8443" which should mean rewritten http-ulrs should be send to https://dom.domain.com:8443/ instead. (Then I think URLs generated via JavaScript are not affected by this setting.)
Without digging deeper into the issue I remember there is a “Secure (SSL):” -Setting for each wiki, i.e. for the main wiki at: https://dom.domain.com/xwiki/bin/view/XWiki/XWikiServerXwiki - maybe it helps to set this one to “true” ?
Edit: the error seems to happen when the list of attachments is going to be refreshed after a successful file upload. To refresh the list, XWiki generates a relative URL to /xwiki/bin/get/Some/Page/?xpage=attachmentslist&forceTestRights=1 and it should be up to the browser to complete this URL to an absolute one, especially deciding if it is HTTPS or HTTP. As the page is delivered via HTTPS, it should obviously select HTTPS too. (Unless I am overlooking something …)
As we are fiddling with the browser tools anyway, can you please open the browser JavaScript console while on some wiki page and type in:
XWiki.currentDocument.getURL('get','xpage=attachmentslist&forceTestRights=1')
I get back a relative URL (like "/xwiki/bin/get/Main/?xpage=attachmentslist&forceTestRights=1" ) but maybe this is different in your case?