When seeking help with this issue, you may be asked for some of the following information:
WordPress version 6.6.2
Active theme: CUSTOM(version 1.4.3)
Current plugin: BackWPup (version 4.1.6) PHP version 8.2.24
Error Details
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An error of type E_ERROR was caused in line 42 of the file /www/wp-content/plugins/backwpup/vendor/guzzlehttp/guzzle/src/HandlerStack.php. Error message: Uncaught Error: Call to undefined function GuzzleHttp\choose_handler() in /www/wp-content/plugins/backwpup/vendor/guzzlehttp/guzzle/src/HandlerStack.php:42
Could you please make sure that BWU has enough permission to backup to AWS S3?
The recommended permission by the community you can take a?look here, also in case it still does not work, could you give us a screenshot of the job config?
You may also troubleshoot by deactivating other plugins to see if it helps.