Most likely you were pasting images into notes in earlier versions of Zotero, when it wasn't supported and could break syncing. Zotero 6 will automatically convert pasted images from older versions into a new, supported format when you load each note. but since you're getting an out-of-memory error you'll need to find the notes in question. You might be able to search for <img in the search bar in All Fields & Tags mode. You can also try data:. Make sure you're searching in the library root, and make sure you've emptied the trash.

If you provide a Debug ID for a sync attempt, we might be able to tell you more.