I have several free style projects, some of then run on a scheduled basis and some of them using a post commit hook from Github.
After upgrading to version 1.528 and updating the installed plugins, the automatic some manual builds and all automatic builds started failing, apart from that, another exception it's raised when trying to open the changelog for a build. Here are the exceptions:

1) On the builds that fail here is the exception log:
tarted by user Federico Saravia
Building in workspace /var/lib/jenkins/jobs/test-api/workspace
Checkout:workspace / /var/lib/jenkins/jobs/test-api/workspace - hudson.remoting.LocalChannel@6281d31b
Using strategy: Default
Last Built Revision: Revision e87ed66aeab0ed5b9da96c574c3c326e4a70e332 (origin/development)
Fetching changes from 1 remote Git repository
Fetching upstream changes from origin
Commencing build of Revision 81206b9ccedc3eeaa1890dc4e960359ad989202d (origin/development)
Checking out Revision 81206b9ccedc3eeaa1890dc4e960359ad989202d (origin/development)
FATAL: hudson/tasks/Mailer$UserProperty
java.lang.NoClassDefFoundError: hudson/tasks/Mailer$UserProperty
at hudson.plugins.git.GitChangeLogParser.parseCommit(GitChangeLogParser.java:70)
at hudson.plugins.git.GitChangeLogParser.parse(GitChangeLogParser.java:59)
at hudson.plugins.git.GitChangeLogParser.parse(GitChangeLogParser.java:23)
at hudson.model.AbstractBuild.calcChangeSet(AbstractBuild.java:922)
at hudson.model.AbstractBuild.access$600(AbstractBuild.java:105)
at hudson.model.AbstractBuild$AbstractBuildExecution.defaultCheckout(AbstractBuild.java:681)
at jenkins.scm.SCMCheckoutStrategy.checkout(SCMCheckoutStrategy.java:88)
at hudson.model.AbstractBuild$AbstractBuildExecution.run(AbstractBuild.java:581)
at hudson.model.Run.execute(Run.java:1603)
at hudson.model.FreeStyleBuild.run(FreeStyleBuild.java:46)
at hudson.model.ResourceController.execute(ResourceController.java:88)
at hudson.model.Executor.run(Executor.java:247)
Caused by: java.lang.ClassNotFoundException: hudson.tasks.Mailer$UserProperty
at org.apache.tools.ant.AntClassLoader.findClassInComponents(AntClassLoader.java:1365)
at org.apache.tools.ant.AntClassLoader.findClass(AntClassLoader.java:1315)
at org.apache.tools.ant.AntClassLoader.loadClass(AntClassLoader.java:1068)
at java.lang.ClassLoader.loadClass(ClassLoader.java:356)
... 12 more

On the changelog screen the raised Exception is different (only if there has been changes in the build), the exception is attached.

Another thing, maybe related to the issue is that jenkins tells me that I have old or unreadable data:
hudson.model.FreeStyleProject api NoClassDefFoundError: hudson/tasks/Mailer$UserProperty

I can delete that data, but every time one of the builds fail it comes back.
I may point that I never installed or used the Mailer plugin. The only plugins I installed and use are the Git plugin and the SSH plugin, so I don't know where this Mailer$UserProperty came from.

I already tried downgrading the plugins to their bundled version but that didn's solve the problem