Something I have noticed previously, is that after some Developer inactivity (say you don’t run the service for 30 minutes) you get this exception. If you then disconnect and reconnect the Developer session to the IS, the service executes OK.
This may not be the problem in your case as you don’t say if you have ever SUCCESSFULLY run your postString service, but it’s definately something to be aware of.
If you’ve never successfully run the service, the most likely cause is incorrect port or ACL configuration. Make sure the username/password you are using is assigned to the ACL applied to your acceptString service and that this service is in the Allow List for the port you are using in the HTTP service call.
Also, if the input variable of the acceptString service is a String called “input”, make sure the data/args document (which is defined in the HTTP service input) has a child node of type string called “input”. Then make sure you map whatever data you want to pass into this variable into it.
Hope something here helps 
Andy Withers
Solutions Architect
Glue Ltd:
www.glueltd.com
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