Marshal data too short accessing mavengems

This seems to be a recent issue, possibly related to some unknown update to rubygems.org:

[INFO] --- maven-jar-plugin:3.0.0:jar (default-jar) @ jruby-stdlib ---
[INFO] Building jar: /Users/headius/projects/jruby/lib/target/jruby-stdlib-9.2.13.0-SNAPSHOT.jar
[INFO] 
[INFO] -----------------------< org.jruby:jruby-tests >------------------------
[INFO] Building JRuby Integration Tests 9.2.13.0-SNAPSHOT                 [4/4]
[INFO] --------------------------------[ jar ]---------------------------------
Downloading from mavengems: mavengem:http://rubygems.org/rubygems/rspec/3.9.0/rspec-3.9.0.pom
ArgumentError: marshal data too short
          load at org/jruby/RubyMarshal.java:147
  marshal_load at uri:classloader:/nexus/rubygems_helper.rb:58
    initialize at uri:classloader:/nexus/dependency_data_impl.rb:33
Downloading from central: https://repo.maven.apache.org/maven2/rubygems/rspec/3.9.0/rspec-3.9.0.pom

This happens during JRuby's build using mvn package -Pbootstrap which downloads and installs additional gems for local development purposes.

The URL in question is clearly not something that rubygems.org supports, and produces a page with the only content as "404 Not Found" when accessed.

Perhaps this page's content is being used as the pom? Or used as some RubyGems data that it tries to marshal and fails?

This is preventing all JRuby CI builds from working. I wiped out my local .m2/repository and could reproduce locally as well.