I just had the same problem. I also use IONOS test cloud. I installed a new installation and made a new bucket for it on IONOS. Images wouldn't upload. However when i use the first bucket i created it was possible to upload the logo and profile/album images. Maybe it's because of the Test mode. I guess i'll will find out when i activate my account.

Beside this IONOS is much easier to setup than amazon/wasabi in my experience.

1. create bucket
2. write down the key and secret key
3. choose your region e.g. s3-eu-south-2.ionoscloud.com don't use the ` when you place this in Studio/settings > endpoint.