Can't upload

www.the-rock.org.uk
Sermon Speaker 4.5
Template: Yoo - Showroom
Joomla 2.5.14

I have been uploading weekly sermons to amazonaws.com now for a good few weeks, but suddenly this week it won't upload.

I've tried uploaded a file I have previously uploaded successfully, but that doesn't work either.

Everything looks fine with the file uploading, but just as it finishes I get an error message saying 'Upload Error 500'.
Can anyone help get me back up and running?

Error 500 usually means that the server terminated the script for some reason. The server error log would tell you why.
Usually, it's caused by upload restrictions, like to big files.

Thanks for the swift reply Thomas!

I've tried uploading a real small sized file, just a few kb, to check its not a size issue, and I get an error 401 with that. Does that mean anything different?

Yep, 401 means something different: "Unauthorized". See en.wikipedia.org/wiki/List_of_HTTP_statu...des#4xx_Client_Error

I don't know however why you see this error. SermonSpeaker doesn't use this one.
Did you check your server log for errors? That's the first one to check, especially for the 500 (Internal Server Error / Fatal Error).

This is the log from the error_log

[09-Sep-2013 18:10:15 UTC] PHP Warning: S3::getBucketLocation(rocksermons): [RequestTimeTooSkewed] The difference between the request time and the current time is too large. in /home/therock/public_html/administrator/components/com_sermonspeaker/s3/S3.php on line 598

[09-Sep-2013 18:10:15 UTC] PHP Warning: S3::putObject(): [RequestTimeTooSkewed] The difference between the request time and the current time is too large. in /home/therock/public_html/administrator/components/com_sermonspeaker/s3/S3.php on line 358

Can you shed any light on what causes this error?

Sounds to me like an error from AmazonS3 which says that the request took to long to process.
If you want to upload a file to AmazonS3 using SermonSpeaker, it will first upload the file to your server and from there it will be stored to the AmazonS3 servers. This error occurs between your server and AmazonS3, not between your computer and the server.

Why that happens is up to speculations for me.