Oracle released changes on the My Oracle Support portal and unfortunately broke the integration with MOS (My Oracle Support).
This is publised on the MOS Knowledge Base Note: Recent Changes to MOS will Disable Enterprise Manager Online Connections (Doc ID 2664002.1). There is a bug opened for this issue Bug 31294163: EM jobs fail with Invalid JSON Response from My Oracle Support. At this time, there's no ETA for a fix.
The workaround is to manually download the catalog and patches and do this work manually.
Meanwhile, you'll have to keep seeing these messages on your Enterprise Manager job list:
