I spent some time investigating this problem for you. I didn't see any errors in our logs indicating that the upload failed but there may have been a problem trying to send the file. If you click on the "Files" tab (to the left of "Send") on the Acrobat.com website and look under "Acrobat.com Files" do you see the file you uploaded there? By the way, there may be more than copy of the file because you tried to upload/send it multiple times.
If the file is there, select it and then click theSend button (between download and delete). This will bring you back to the Send page with the file already selected. Now try sending the file again. Hopefully with the file already uploaded it will work.
Another option is to try using another web browser such as Firefox or Chrome.
It looks like a small number of users are experiencing this problem but we're still unable to reproduce the problem on our end. We will keep looking into the issue and hopefully have a solution soon. Please let us know if either of the workarounds provided work for you.
I have the same problem since conversion. Every time I try to send files I get the error message that there is a problem uploading one or more files. I followed your instructions above and the one file that was not uploaded was then uploaded and all were sent. This solved my problem temporarily; however, if there is not a permanent fix I will not renew my subscription in January.