Internal Server Error returned from IQueryRestService.getScopedQueries

All,

our version 5.0 server is showing strange behavior on requests to IQueryRestService.getScopedQueries: Regular users run into 500 Internal Server Error whereas repository admins still get the expected response. To make matters worse, I did not find any trace of this behavior in the server log files (neither ccm.log, nor any of the files in tomcat/logs). Also, this used to work in the past, the failing behavior was first observed on Jan, 9th.

I tried to get more details by writing a small test case which performs a GET to the REST service:

        ITeamRawRestServiceClient rawRestServiceClient = repository.getRawRestServiceClient();
        URI scopedQueriesUri = new URI("service/com.ibm.team.workitem.common.internal.rest.IQueryRestService/scopedQueries?scope=3&includeCount=false&includeParentScopes=true&projectAreaItemId=...some_uuid...&suppressCrossRepoQueries=false");
        IRawRestClientConnection connection = rawRestServiceClient.getConnection(scopedQueriesUri);
        Response response = connection.doGet();

but it does not provide more details than were known before:

com.ibm.team.repository.common.transport.TeamServiceException: CRJAZ1247E The request to the server failed.  The server returned the HTTP error 500 with this error text: Internal Server Error.
    at com.ibm.team.repository.transport.client.ClientHttpUtil.throwHttpClientError(ClientHttpUtil.java:1445)
    at com.ibm.team.repository.transport.client.ClientHttpUtil.executeHttpMethod(ClientHttpUtil.java:526)
    at com.ibm.team.repository.transport.client.ClientHttpUtil.executeHttpMethod(ClientHttpUtil.java:322)
    at com.ibm.team.repository.transport.client.ClientHttpUtil.executeHttpMethod(ClientHttpUtil.java:220)
    at com.ibm.team.repository.transport.client.ClientHttpUtil.executeHttpMethod(ClientHttpUtil.java:229)
    at com.ibm.team.repository.transport.client.RestClientConnectionBase.executeMethod(RestClientConnectionBase.java:324)
    at com.ibm.team.repository.transport.client.RestClientConnectionBase.doMethod(RestClientConnectionBase.java:187)
    at com.ibm.team.repository.transport.client.RestClientConnectionBase.doGet(RestClientConnectionBase.java:122)
    at com.ibm.team.repository.transport.client.TeamRawRestServiceClient$RawRestClientConnection.doGet(TeamRawRestServiceClient.java:96)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:60)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:37)
    at java.lang.reflect.Method.invoke(Method.java:611)
    at com.ibm.team.repository.client.internal.ServiceInterfaceProxy.invokeServiceCall(ServiceInterfaceProxy.java:254)
    at com.ibm.team.repository.client.internal.ServiceInterfaceProxy.invoke(ServiceInterfaceProxy.java:110)
    at com.ibm.team.repository.client.internal.RawRestServiceClientProxy$RestClientConnectionProxy.invoke(RawRestServiceClientProxy.java:121)
    at com.sun.proxy.$Proxy9.doGet(Unknown Source)
    at TestGetScopedQueries.testGetScopedQueries(TestGetScopedQueries.java:26)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:60)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:37)
    at java.lang.reflect.Method.invoke(Method.java:611)
    at junit.framework.TestCase.runTest(TestCase.java:168)
    at junit.framework.TestCase.runBare(TestCase.java:134)
    at junit.framework.TestResult$1.protect(TestResult.java:110)
    at junit.framework.TestResult.runProtected(TestResult.java:128)
    at junit.framework.TestResult.run(TestResult.java:113)
    at junit.framework.TestCase.run(TestCase.java:124)
    at junit.framework.TestSuite.runTest(TestSuite.java:232)
    at junit.framework.TestSuite.run(TestSuite.java:227)
    at org.junit.internal.runners.JUnit38ClassRunner.run(JUnit38ClassRunner.java:83)
    at org.eclipse.jdt.internal.junit4.runner.JUnit4TestReference.run(JUnit4TestReference.java:49)
    at org.eclipse.jdt.internal.junit.runner.TestExecution.run(TestExecution.java:38)
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:467)
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:683)
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run(RemoteTestRunner.java:390)
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner.java:197)

What could cause this problem for regular users, while getScopedQueries still works for admins?
Where/how can server-side logs for this problem be found/generated?
Any suggestions what else to do?

Thanks in advance!
Markus