I suddenly started experiencing this problem too;
[Pipeline] End of Pipeline
java.lang.UnsupportedOperationException: Refusing to marshal org.codehaus.groovy.runtime.GStringImpl for security reasons
Everything was working fine until we updated the git plugin, which seemed to update various other things. Now we see this error. This is some sort of regression which has completely crippled our automations.
I even installed a fresh lts jenkins master (2.60.1) chose the recommended list of plugins and only installed the artifactory plug in on top of that and am seeing the same problem. Please fix ASAP.