Errors occuring with Assets and Amazon S3
There are a few regions that support only S3's v4 of signing requests without the possibility of falling back to v2.  The S3 library that Craft 2 uses only supports S3 v2. Check this table for reference - if it says "Version 4 only" in the last column then that region can't be used in Craft 2.

We're not switching out that library in Craft 2.x, but it won't be a problem in Craft 3, so currently your best bet is to move your bucket to a different region, which also supports S3 v2 signing of requests (any other one, really).