After migrating to M6 from M5 I could only login as the ADMIN user both from the eclipse client and WEBUI. This is a fresh server installed on M5 (had not been previously migrated). We are using DB2 as a backend and Tomcat for the server. We did implement the "Manage Jazz Security" steps in the /JazzTeamServer-1.0M5/jazz/server/index.html file to ensure secure access only.

Tried to follow the tip from a previous post where it was recommended to add Jazz roles to the previous users from the admin UI, but got this error on the UI:
Error fetching user data from the user registry. Please check your registry configuration.

Bad Request

Also got this in the tomcat.log on the server:
22:33:11,562 WARN .team.repository.servlet.AbstractTeam
ServerServlet - UnknownUserException processing GET request for com.ibm.team.pr
ocess.internal.common.service.IProcessRestService.getContributorByUUID(). The re
quest was made by user "ADMIN" from "9.67.90.142". The stacktrace hash is B6BE5C
18CEB4E6076BAE847AD2B928472E5F8328.
com.ibm.team.repository.common.UnknownUserException: mmallo
at com.ibm.team.repository.service.internal.userregistry.TomcatUserRegis
try.findUser(TomcatUserRegistry.java:128)
at com.ibm.team.repository.service.internal.userregistry.TomcatUserRegis
try.fetchGroupsForUser(TomcatUserRegistry.java:143)
at com.ibm.team.repository.service.internal.userregistry.ExternalUserReg
istryService.fetchGroupsForUser(ExternalUserRegistryService.java:190)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.
java:64)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAcces
sorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:615)

FYI
I continue through with the new M6 setup screens at https://localhost:9443/jazz/setup and created a new test user that could access jazz fine, but still the previous users were not able to log in.