Every authenticated request's session will be logged out by default (at the end of that request) and further requests will require re-authenticating.
In order to keep the session alive across multiple requests -
Append the parameter '_logoutSession=false' to the request URL
http://.../ecosys/api/...&_logoutSession=false
For SOAP requests, set the 'LogoutSession' token to 'false'
<SampleOptions>
. . .
<LogoutSession>false</LogoutSession>
It is recommended to re-use the same session across multiple requests to optimize server performance.