Please try to see if the backend.log provides more helpful information to troubleshoot the error.
You can find it in Administration -> Maintenance -> Log Files or you can reach it in the data_dir/run folder.
The fact the credentials are working for S3 is good but might not be related. Make sure those creds are granted access to DynamoDB as shown here.
If still failing, try to remove credentials from the plugin and flag the option use credentials from environment instead. In that case you will need to authenticate to AWS from the command line, using the same account that runs DSS. have a look here and here for more info on that.
If it works this way, then the issue is on the plugin and we'll have a look into it.
Architect @ Dataiku