"Test connection" on the Configure System page fails with this client error:
Client error - Could not send message
On the same maschine a test with a standard user and curl was successfull.
Please note: The jenkins server is behind a proxy.
But with user jenkins curl failed (no proxy setting in jenkins environment, this technical user hasn't a bash)
If I use the -E option curl is successfull (because the proxy settings are taken).
A proxy is set in the advanced plugin settings in jenkins.