The error message tells you how to fix the problem (which is from dragging an image into a note, which isn't allowed):

The note '<p><img src="data:image/png;base64,iVBORw0KGgoAAAA…' in your library is too long to sync to zotero.org.

Search for the excerpt above or copy and paste 'DEJ559EM' into the Zotero search bar. Shorten the note, or delete it and empty the Zotero trash, and then try syncing again.

It depends where the image is coming from. Copying an image from a web page might cause it to be embedded with an HTTP URL in the <img> src attribute, which is OK (though Zotero will need to load the image from the remote server every time you view the note), but if the image gets inserted as a data URI (as above) it will likely break syncing.

The safe way to add an image to an item is to add it as an attachment.