Description of my issue:
I am unable to back up to Amazon S3... getting the 'The request signature we calculated does not match the signature you provided. Check your key and signing method." Message.
I found a post from a couple of years ago in the forum and I checked Disable multipart uploads.
I had this problem once before and I can't remember the solution. It may be relevant that I just changed hosts from Rochen to Wiredtree.
Link to log: http://megmccarthy.com/AkeebaBackupDebugLog.txt