Note that the url from the error message does not seem to be correct.

https://watlbugzilla01.watson.ibm.com/xml.cgi?id=8589
produces a "404: The requested URL /xml.cgi was not found on this server."

The actual url of the associated bug is:
https://watlbugzilla01.watson.ibm.com/show_bug.cgi?id=8589

After installing the latest version of stand alone RTC client, I get a more descriptive error.

"Could not create work item from bug 9091: User "tschmidt" does not have the required permission to perform this operation. The "JazzAdmins" role is required to perform this operation."

Now I simply need to find the admin or owner of https://jazz.net/sandbox02-ccm, in order to request such access.

Let me assure you that when (if) you deploy RTC yourself the Bugzilla Importer does work given one has JazzAdmin permission.

What is likely failing is the attempt to add a new RTC user to associate with various attributions (ownership, creator, etc) of the data being imported.

Here's a suggestion:  I think the Import wizard allows you to save the Bugzilla as an XML file local to you.  Let it do that, then change all things that look like a 'contributor' to your ID, then try the import again.

I don't know if JazzAdmin will be allowed on that repository

We have used the facility since RTC v1.0 for various purposes.