Actualizo con lo que se va comentando en el mailing list del DNS-OARC (Domain Name System Operations, Analysis, and Research Center):
The chainof keys was always correct (unlike many DNSSEC problems, the DS, and DNSKEY were always in sync), the problem being that ZSK 52263 produced invalid signatures.
Two hypothesis:
1) Something strange in this specific key broke the signatures (funny but unlikely)
2) The signing system had a sudden problem. Note that .ru went back, not only to the the previous ZSK but also to a previous zone, and the SOA serial (4058856) did not change since (it changed very ~ two hours before). It is possible that they cannot sign anymore.
Note: there will be a short talk about this incident in FOSDEM (Brussels) on saturday, either at the DNS devroom or during the lightning talks.
Vamos, que aún no está claro cómo sucedió el problema con la clave de firma. No obstante, el problema quedó solucionado.