Found following error in ZSS STC log:
ZWES1014I ZIS status - 'Failure' (name='ZWESIS_STD ', cmsRC='47', description='ZVT is NULL', clientVersion='2')
ZOWE 2.4.0
Solution is to start the cross memory server (/S ZWESISTC) before starting the main STC (/S ZWESLSTC).