HTTP POST https://api.zotero.org/users/5423684/items/6XSDE9SE/file failed with status code 400:

Files above 4 GB are not currently supported

You have a file larger than 4 GB in your library, which can't currently be synced. Paste "6XSDE9SE" into the search bar in All Field & Tags mode to find the attachment in question, delete it, and empty the trash. You can then add it back as a linked file if you want.