It seems that the issue you are facing is related to an open ticket: RTFACT-6759.
The embedded Groovy may have problems when @Grapes finds dependencies that are already included in Artifactory itself.
In order to solve this issue you can remove the use of @Grapes and instead place your libraries:
dforg.codehaus.groovy.modules.http-builder
oauth.signpost, module
oauth.signpost, module
under $ARTIFACTORY_HOME/etc/plugin/libs
You will need to upgrade your Artifactory to our latest version because the ability to load libraries from the libs directory was added recently. (RTFACT-6562).