Hi @Valentin ,
how do you upload file?
Regarding the specific situation of "it's never synchronized", please provide some screenshots.
There are two ways can upload file to SharePoint via Teams.
Way one: Teams-->Files-->Upload
Uploading an empty file this way does not give an error.
Way two: Teams-->Posts-->Attach
Uploading an empty file this way will give an error.
Just add something and reupload.
Then upload successfully.
Finally, You can see it in SharePoint.
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