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The problem can be caused by using an alias address for your JIRA server or using "http://" address instead of "https://" in your connection settings. In these cases JIRA server will redirects redirect all requests, sent from JIRA Client to the alias address, to the main address. This will work most of the time, but may fail in case of upload.
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So to avoid this, make sure you are using the main JIRA URL in your connection settings - the one that you see in the browser when you are logged in.
If you realize your connection does use the wrong address, you can create a new connection with the correct address and simply move your queries from the old connection using drag-and-drop and then delete old connection. Alternatively you can change the existing connection settings, which requires a little more effort. Please see how to do it here.