Due to bug 49217 when trying to preview content with a reference we get a nasty error rendered in the response.
We may have to workaround yet another API bug here as I suspect bug 49217 will be RESOLVED WONTFIX as is usually the case when I raise bugs for the API.
To replicate click edit
Replace textarea content with
<ref>[http://www.priberam.pt/DLPO/default.aspx?pal=saudade Dicionário Priberam da Língua Portuguesa]</ref>
and hit preview.