I set "Store error text in" to a variable and it doesn't return anything. So it doesn't appear the Web Call Service errors out, it just doesn't execute for some reason. Hopefully we determine it's a server side issue today.
does it really return nothing?
if it reached the service it should either return GUID of started workflow or an error message.
can you post how do you build service URL (you need not post host name nor full site path)
do use some variables to build up the path?