We seem to have a very similar issue to the one reported here:
Q2 2010 (Version 2011.2.713)
Problem: RadAsyncUpload sometimes fail to upload the files to the target folder.
except the version we are using is 2013.1.417.45 in which the issue is supposed to be fixed. The challenge is that the problem is not consistent. We're able to upload video files usually without a problem but at the same time we have many users, dozens a day, that get an error during the postback portion of the upload when the temporary file is not found. We tried using the custom handler fix as described in that earlier version problem solution but that doesn't seem to affect the issue. The users seem to be using a variety of browsers and systems so we haven't been able to isolate a pattern.
We'd even be okay not having the temporary files delete at all and we'll manually delete them daily if that is possible.
Is there any potential solution to this?