Hello,
Is this issue confined to this single specific CSS file or we face this error when uploading any hosted file ?
If possible, can you please share the CSS file with which you experienced this issue. This would help us debug your issue more appropriately.
In past, we had this issue for some existing very old hosted files whose file name in database was either empty strings "" or NULL. But, you say this is file you were just trying to upload..
Thanks,
Nitin