Why can't we open Testplan via Url with local configuration? Trying to find reason for permission error

Dear all,

When trying to open a test plan with an url like this (local configuration):
https://our.server.com/qm/service/com.ibm.rqm.integration.service.IIntegrationService/resources/ourprojectarea/urn:com.ibm.rqm:testplan:7?oslc_config.context=https://our.server.com/qm/oslc_config/resources/com.ibm.team.vvc.Configuration/_x54_sIo7EeeY3ZG_ZHn1tA

We get the following error message:
{"errorMessage":"CRJAZ1316E The user "username" does not have permission to read item "_SLb5EfvIEeW_g7aHnHYRQA", which has the "ProjectArea" item type.","errorClass":"com.ibm.team.repository.common.PermissionDeniedException"

We can however open the test plan via the Web user interface. We also can open the local configuration. The user has extensive user permissions both on RQM & GC project areas.

It would already help if we knew which item is causing the message: neither project Area nor team area nor test plan nor global or local configuration has this URI (_SLb5EfvIEeW_g7aHnHYRQA).

The syntax of the url is most likely OK as the code that calls this url works on 2 other systems without trouble.

Thank you in advance for any ideas how to solve this problem.