It exists apple security updates, which block the Java Web Start for older Java versions. In order to unblock the Java Web Start, you'll need to update you Java version.
If this doesn't work for some reason, changing the “Blacklist”, responsible for the blocking, may help. To do so, you will need to change the file “XProtect.meta.plist” on your systems.
To activate the blacklist again and redo your changes, you can simple delete this file. Your Mac will create this file with the latest information after a restart.
You can delete the file by opening the terminal and typing the command: sudo rm /System/Library/CoreServices/CoreTypes.bundle/Contents/Resources/XProtect.meta.plist
The GECAMed client should start now on this machine as usual again.
You can also use this script: fix_java_on_mac_osx.zip
It will replace the XProtect.meata.plist file with a different one, that includes the changes.
In order to use this script, you need to download and unpack the ZIP-file and execute the command-file in the unpacked folder with a user whith admin privileges. Be sure, that the file XProtect.meta.plist is in the same folder as the file fix_java.command when executing it. Read the warning and confirm it, to run the fix. You will need to enter your system user password.
If the script executes with “SUCCESS!” your GECAMed should start on this machine as usually again.