Why am I suddenly getting this error when trying to build from Jenkins?

After restarting our Jenkins system for an administrative reason all our builds are now failing with this error:

Started by user SVC_Jenkins
[EnvInject] - Loading node environment variables.
Building on master in workspace /var/lib/jenkins/jobs/remote-administration-monitor/workspace
RTC : checkout...
RTC : Build initiated by request from RTC
FATAL: RTC : checkout failure: null
java.lang.UnsupportedOperationException
    at java.util.Collections$UnmodifiableCollection.add(Collections.java:1075)
    at com.ibm.team.build.internal.hjplugin.RTCScm.checkout(RTCScm.java:1041)
    at hudson.model.AbstractProject.checkout(AbstractProject.java:1269)
    at hudson.model.AbstractBuild$AbstractBuildExecution.defaultCheckout(AbstractBuild.java:607)
    at jenkins.scm.SCMCheckoutStrategy.checkout(SCMCheckoutStrategy.java:86)
    at hudson.model.AbstractBuild$AbstractBuildExecution.run(AbstractBuild.java:529)
    at hudson.model.Run.execute(Run.java:1738)
    at hudson.maven.MavenModuleSetBuild.run(MavenModuleSetBuild.java:531)
    at hudson.model.ResourceController.execute(ResourceController.java:98)
    at hudson.model.Executor.run(Executor.java:410)
ERROR: RTC : checkout failure: null
An attempt to send an e-mail to empty list of recipients, ignored.
Finished: FAILURE

We are using:
Jenkins 1.651.2
RTC Build Toolkit 4.0.2
Team Concert Plugin 1.1.9

When we test the connection to RTC through the Jenkins dashboard we see "Connection test was successful"

Our sys admin is out for medical reasons so I'm trying to help out with getting the builds up and running again. Did we not restart everything we need to? Any particular service we should check?

Thanks in advance!