I am not sure, if protocol-relative url in the sidebar works, when it works, please change all sitesupport-url to protocol-relative urls, so https user get a https link. When not, we need another bug for that, which blocks this bug.
This needs also some on-wiki changes, because at least de.wp has a local message [[de:MediaWiki:Sitesupport-url]].
Thanks.
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Severity: normal