Yesterday I could not use Adobe Send the way I always have before. I usually scan in my file, bring up the saved PDF, click "Tools" then "Send Files", add the recipient's email, then click on "Create File". Once my file uploaded to the Cloud, a pop-up would let me write a message then I clicked to send. The pop-up would change and I clicked "Close" and I was done. Yesterday the pop-up was different, showing an Adobe page as though I had never uploaded to the Cloud at all. I ended up uploading again using the information showing on that pop-up. It worked but my client had some trouble opening the file. Is this temporary? I hope so.
Are you using Send & Track service online via web browser https://cloud.acrobat.com/recent or using desktop application Adobe Reader DC? There isn't any changes applied to the Send & Track services recently. The document should have been uploaded to the Document Cloud in same order as it used to.
It could be an intermittent issue with the cloud services. You may check it once again and see if you have been able to upload the file now. If not, then you may try with a different web browser.