AWS ElasticBeanstalk NodeJS Application version cannot be uploaded/deployed/reverted "Unable to create the application version:" / "Validation error"

I have a NodeJS application running via ElasticBeanstalk, have deployed 59 previous application versions via the "upload and deploy" button in the EBS dashboard for the application environment. Before, I uploaded a zip file and the environment updated successfully (all at once strategy), never had issues.

• Trying to upload a zip file the last 2 days (for the first time in some months) fails, after uploading the "Upload and deploy" popup shows only a red X icon with text "Unable to create the application version: ", without further information.
• The "Application versions" page of the environment shows errors on loading like "Validation error" and "Error Unable to retrieve default rolling update values, leave the options blank to retrieve the defaults:", but is otherwise functional.
• Reverting to a previous application version via the "Application versions" page also no longer works. Just like when uploading a new version, when reverting the "Deploy Application Version" popup shows "Unable to create the application version: ", without further information.
• Deleting previous application versions errors out frequently, but does succeed after several attempts, I've deleted most of my old versions to make sure I'm under the limit.

Is there a problem with the service currently, or has something changed in the past months requiring changes on my end? Is there any way I could debug this, as the "Upload and deploy" popups provide no information about problems?

• I also cannot create a new (NodeJS) application environment via EBS dashboard, when creating a new environment from an application or from the environments page, it fails and shows only "Validation error". Update: After trying multiple times, it does succeed eventually.