We have a datastore (sde connection) created on Portal. The "layers" tab is supposed to display all feature/map services created from the sde connection. For some reason, the "Layers" tab is not responding and nothing is displayed:

We've tried to reboot the server and Portal machines and stop/start the system publishing tool but didn't fix anything. Does anyone know how to fix this issue? BTW, the ArcGIS Enterprise version is 10.9.1.