Kirk, I just updated PM to 4.6.4.1 and started getting upload errors when uploading to Smugmug tonight. Some photos would upload and some would give me an error message and some said "stalled". If I tried uploading the photos that got an error message the first time, some would load and some gave me the error message again. I had to keep trying and finally got all the files to upload.
When the files upload it's very slow and the error message ends with write invalid argument. Not sure if this is a problem with the new version of PM or a problem on Smugmug's end.