This issue appears locally on SEPM as well as accessing the console through Web Console or Remote Java Console if this is done from polish operating systems.
It was found that the problem is present only when 2 specific views are selected on "Clients" tab - System klienta (Client system) or Stan klienta (Client status). While selecting "Clients" tab (Klienty) there is never ending "Loading" pane (Trwa ładowanie).
The issue is scheduled to be fixed in the Maintenance Patch 1 for current version of the SEPM. Currently there is a patch fixing the issue in 12.1 RU1 versions.
In order to install the patch follow the steps:
1. Warning! The .jar file can be used only in the localised Polish language version of SEPM
2. Save the attached scm-ui-res.jar file
3. Close all local, remote and java consoles of SEPM (in case of replication enabled, all consoles connected to all SEPMs should be closed)
4. Stop the Symantec Endpoint Protection Manager service (in case of replication enabled, the service should be stopped on all SEPMs)
5. Make a backup of the original smc-ui-res.jar which default location is \Program Files\Symantec\Symantec Endpoint Protection Manager\tomcat\webapps\ROOT\clientpkg
a. Warning! The backup should be made outside of the original folder
6. Replace the original smc-ui-res.jar file with the one downloaded from the document
7. Clear Java cache:
How do I clear the Java cache?
8. Start the Symantec Endpoint Protection Manager on each server
9. Run the SEPM console and switch to Client tab to verify if the issue was resolved