I'm trying to upload a small (22Kb) xlsm file in reply to a thread, but after I've hit the upload button there's a red warning icon next to the file, which if I hover over it gives me the message:

"403 [IOErrorEvent type = "ioError" bubbles=false cancelable=false eventPhase=2 text="Error #2038"]"

I tried to upload a screenshot of the error, but ... well, you can see where this is going, can't you?