Error when selecting S3 Bucket

Reproduce the failure by creating a job, selecting S3, entering AWS credentials, and selecting a bucket. Start with vendor/aws/aws-sdk-php/src/Handler/GuzzleV6/GuzzleHandler.php and vendor/guzzlehttp/promises/src/Promise.php, using the supplied stack trace to follow the error path. Done means bucket selection works and the settings can be saved without the fatal error.

Describe the bug

I get this when I want to select a s3 Bucket after introducing my AWS keys

Fatal error: Uncaught TypeError: Argument 1 passed to Aws\Handler\GuzzleV6\GuzzleHandler::Aws\Handler\GuzzleV6\{closure}() must be an instance of Exception, instance of Error given, called in /var/www/web/wp-content/plugins/backwpup/vendor/guzzlehttp/promises/src/Promise.php on line 203 and defined in /var/www/web/wp-content/plugins/backwpup/vendor/aws/aws-sdk-php/src/Handler/GuzzleV6/GuzzleHandler.php:45 Stack trace: #0 /var/www/web/wp-content/plugins/backwpup/vendor/guzzlehttp/promises/src/Promise.php(203): Aws\Handler\GuzzleV6\GuzzleHandler::Aws\Handler\GuzzleV6\{closure}() #1 /var/www/web/wp-content/plugins/backwpup/vendor/guzzlehttp/promises/src/Promise.php(156): GuzzleHttp\Promise\Promise::callHandler() #2 /var/www/web/wp-content/plugins/backwpup/vendor/guzzlehttp/promises/src/TaskQueue.php(47): GuzzleHttp\Promise\Promise::GuzzleHttp\Promise\{closure}() #3 /var/www/web/wp-content/plugins/backwpup/vendor/guzzlehttp/guzzle/src/Handler/CurlMultiHandler.php(118): GuzzleHttp\Promise\TaskQueue->run() #4 /var/www/web/wp-co in /var/www/web/wp-content/plugins/backwpup/vendor/aws/aws-sdk-php/src/Handler/GuzzleV6/GuzzleHandler.php on line 45

To Reproduce
Steps to reproduce the behavior:

1. Create a Job
2. Select s3 as destination service.
3. introuduce your aws credentials.
4. Try to select a bucket.

Expected behavior
I expected to be able to select a bucket and then save my settings.

System (please complete the following information):

• OS: Linux
• Version Ubuntu 20.04

Additional context
Add any other context about the problem here.