After upgrading jenkins and updating the gerrit trigger plugin, it couldn't read the gerrit-trigger.xml file so I deleted it. Now when I try to add a new server under gerrit trigger or try to edit existing server, when I save it throws an exception:

Caused by: java.lang.UnsupportedOperationException: Refusing to marshal com.sonymobile.tools.gerrit.gerritevents.watchdog.WatchTimeExceptionData for security reasons; see https://jenkins.io/redirect/class-filter/
    at hudson.util.XStream2$BlacklistedTypesConverter.marshal(XStream2.java:530)
    at com.thoughtworks.xstream.core.AbstractReferenceMarshaller.convert(AbstractReferenceMarshaller.java:69)
    at com.thoughtworks.xstream.core.TreeMarshaller.convertAnother(TreeMarshaller.java:58)
    at com.thoughtworks.xstream.core.AbstractReferenceMarshaller$1.convertAnother(AbstractReferenceMarshaller.java:84)
    at hudson.util.RobustReflectionConverter.marshallField(RobustReflectionConverter.java:265)
    at hudson.util.RobustReflectionConverter$2.writeField(RobustReflectionConverter.java:252)

So following what is written on https://jenkins.io/redirect/class-filter/ I am reporting this issue. Note that after doing save a few times, little by little it saved all the needed values even though this exception continues to occur.