Hola Enrique,
You have some posts in forum talking about this issue:
- DOI registration with Crossref plugin stopped to work in ojs 2.4.7.1 - #2 by marc
- CrossRef plugin submission failure
Long story short, CrossRef discontinued their old API and now OJS 3.1 or older are not able to register directly. I’m not related to PKP or CrossRef so I can tell you more about the reasons why, but you can contact CrossRef if you like and ask them directly.
Fortunately, the manual upgrade is still working (you can export, and upload your xml file to crossRef admin tool).
Right now the recommended solution is upgrade to last stable (3.3 or at least stable-3.2).
Cheers,
m.