Well, I ‘fixed’, by running the builds with jdk 8 instead of 7. As it’s just for javadocs, it’s not a big deal.
So it’s not clear to me what happened.
One theory is that jfrog changed their server configuration, and JDK 7 by default is using a deprecated algorithm.
We had some builds start failing over the last few days. Switching to Java 8 fixed it. I also installed latest OpenJDK 7 JDK ( it had been using oracle jdk7), and switched to that and it also fixed the problem. I stuck with that approach to stay consistent building with a Java 7-based JDK.