We are currently in the process of upgrading our XWiki instance from enterprise 9.4 and tomcat8 to the current available tomcat9 stable release.
Most things seem to work just fine but one of the major problems we are facing is that in the administer wiki section our users and groups are no longer being show and there is only a loading bar.
In the rights section if you sort by users they are properly linked and shown. If you sort by groups and enter a group the users are shown as a HTML string instead of a linked name.
It doesn’t matter if we install a fresh server and migrate the old files to the new one or if we a take clone of the old XWiki instance and upgrade it.
Are the users saved differently compared to previous versions?
Do you reproduce the problem on the fresh instance before migration? I don’t reproduce on a clean instance (13.6) so it must be caused by the upgrade. Just to be sure, did you try to clean your browser’s cache and then reload the Users, Groups and Rights administration sections?
It does seem to work properly in a freshly installed instance. I also cleaned my browser cache in that sadly did not help.
Is it possible that trying to update everything in one huge step could lead to problems?
What can happen with an upgrade from a very old version is that you have some customization of standard pages (done on purpose or by mistake) and the merge did not go that well for those.
What you can do to check that is to go to the Extension administration → switch to Installed extensions → search for XWiki Standard Flavor and ask for “Compute changes” (same buttons group as “Uninstall”). You will get a complete diff between the current state and all the pages of the standard flavor and a button to reset any page which is not supposed to be customized (be careful since is perfectly expected for some pages to be customized, for example the wiki level rights setup in XWikiPreferences).