Looking at your log output, the mediaCache is trying to locate the item here,
Is this where you are expecting it to be looking?
I can see that you have multiple sources configured. It is going through each of the sources in turn to see if it can handle the request. It looks at the beginning of the url (http/) to match the prefix set for the source.
Once if finds a source that can handle the request, it won't try other sources. It then tries to make a connection to the source and do a HEAD request on the file and get the file length.
. . .
It is receiving a HTTP 500 response from this request.
MediaCacheItemHTTPImpl.init hostId:boxer url:http://boxer:3030/uploads/abcdefstaging/52faf83d1be8bdb24d000001/_OneMintest.mp4?...
It tries 3 or 4 times before giving up.
The ELB in front of Wowza shouldn't cause an issue. A 500 response comes from the web server that the Media cache is talking to. It has received the request but for what ever reason, it cannot handle the request. It is normally due to a misconfigured web server or some coding error in the web server application that handles the request. It could also happen if one of the values that you are sending is not the right type, i.e. it is expecting a number and you are sending a string value.
If you look at the web server error logs, it may give some indication as to what went wrong. If there is also an ELB in front of the web server, I guess that could also cause issues if the ELB thinks it cannot handle the request from the Media Cache, especially, if it is sending it out to the wrong server.
Note: I wrapped your logging output with code tags so that it is easier to read.