When an unregistered user attempts to add an external link (either directly or through a template transclusion), nothing changes on page submit, i.e. a click on the "Publish changes" button simply reloads the edit interface. It is not possible to proceed further unless the link or template is removed.
It seems to be caused by 0a221920ae9b9d00725a9b822d73c323f85f890b/T275710: Deprecate IContextSource::getWikiPage and IContextSource::canUseWikiPage in the ConfirmEdit extension. A Captcha is supposed to be displayed, but nothing happens on submit.
Reproduced on test.wikipedia.org. Affects 1.37.0-wmf.12.