I compress my movie and I do all the LAFCPUG meeting movies in either Compressor or Episode sometimes Adobe Media encoder and then open the YouTube page in Safari an open the upload video page and the drop in my file. I use a 1920 X 1080 .H264 file in most cases. Works great.
I was seeing your problem happen a lot to my customers trying to post to You Tube from iMovie not from FCPX but anything is possible.
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Dean
"When I see you floating down the gutter I'll give you a bottle of wine."
Captain Beefheart, Trout Mask Replica.