I am having problems in my job configuration where RTC validate workspace is failing with the error:
Credentials chosen do not resolve to valid credentials. Check domain restrictions if the credential is owned by a domain
Further details show the execption
com.ibm.team.build.internal.hjplugin.InvalidCredentialsException: Credentials chosen do not resolve to valid credentials. Check domain restrictions if the credential is owned by a domain.
at com.ibm.team.build.internal.hjplugin.RTCLoginInfo.(RTCLoginInfo.java:86)
at com.ibm.team.build.internal.hjplugin.RTCScm$DescriptorImpl.validateConnectInfo(RTCScm.java:627)
at com.ibm.team.build.internal.hjplugin.RTCScm$DescriptorImpl.doValidateBuildDefinition(RTCScm.java:789)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:606)

I have updated the global configuration page with valid credentials and a Test Connection passes until I reload the Configuration page when the Credentials for RTC reverts back to -none -

I tried several types of credentials (Global, System, and even in a domain) and none of them would stick when saving the Configuration page. I was able to work around the issue, but overriding the RTC configuration in each Jenkins job, but that is not maintainable.

We recently updated the credentials plugin to support connections to GitHub and I suspect the domain support is causing issues somewhere.
We have recreated the issue on 2 servers. One has all the latest plugins available for the LTS version.