That looks like you have a proxy that is not configured correctly!
Does the subtask get created in the back end ok? Or is this happening as soon as you hit "create"?
Ok, then you'll need to look at the proxy logs to find out what it was trying to display, then you can check what JIRA is serving up on that.
However, before doing any of that, do the other basic check. Look at the "base url" in JIRA under "general configuration" in the admin section. Is that correct? And it is the url you are using to get to this JIRA when you are getting this error?
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