hmmmm…
My EC2Instance Roles policy look loike this:
[
{
"Action": [
"s3:PutObject",
"s3:AbortMultipartUpload",
"s3:PutLifecycleConfiguration",
"s3:PutObjectVersionAcl",
"s3:PutBucketCORS",
"s3:DeleteObject",
"s3:PutObjectAcl",
"s3:Get*",
"s3:List*"
],
"Resource": [
"arn:aws:s3:::<data-bucket>",
"arn:aws:s3:::<data-bucket>/*",
"arn:aws:s3:::<backup-bucket>",
"arn:aws:s3:::<backup-bucket>/*"
],
"Effect": "Allow"
}
]
Bucket permissions look like this, but security hub already screams at me, that it´s too open 
I can upload files from ec2 instance without problems with cli (instanceprofile works in that regard)
UPDATE: noticed, that the upload progress reaches 100 %, and only after that I´m getting the error (had to be quick with Greenshot to capture this 
)
hmmmm … seems he was successful uploading the multiparts, but couldn´t finish the process afterwards