Converting images to WebP - Overcoming Timeout Errors?
You might want to check out the discussion on this pull request. Since ImageOptimize uses regular Craft transforms, it's likely that this is something in the AWS S3 plugin.

Indeed, there's an issue 400 Bad Request: token expired that appears to be identical. If there's something I can/should be doing to renew the token, I'll definitely make that change.

I'm pending information at the moment.

EDIT: I believe I have found and fixed the issue in the aws-s3 package from P&T; I created a pull request to address it.