For a detailed description, please see the question I posted at Colloquium (Wikiversity).
Briefly, if you use the Cite Journal template on Wikiversity (but not on Wikipedia), the PMC identifier ("pmc=1234567") produces an erroneous URL that results in a 404 page ("this site can't be reached").
Workaround
The quick workaround is to simply omit links to PubMed Central (PMC) articles until this gets fixed.
An alternative workaround would be to enter references using Cite > Manual > Basic form, i.e., you type out the entire citation yourself, including a properly formatted link to the PMC article.
Of course, if the article you are citing does not have a PMC version, then none of this applies. (Best I can tell, PMID links work correctly.)
Bug Report