I have a similar problem. Noticed a few days ago that things were not syncing whereas they had been -- I am a new use. I have standalone on my laptop, but when in library I prefer to use computer station which has two monitors. Zotero is installed on library computer. I found that for a while I could work on the library computers and add to the installed zotero and it would sync. Similarly, I would add on standalone on my laptop and it would sync with zotero.org. I then saw that as long as zotero.org synced with the library computer, I was then able to sync zotero.org with standalone on my laptop, and all new work then appeared in my zotero standalone on laptop, even though it had originated at zotero.org on the library computer.

This was great; then suddenly it stopped working and now when I work on standalone zotero on my laptop I can't get that to sync. Today I worked on laptop before heading to library computer. Noticed sync arrow on laptop said "last sync 4 days ago". Tried pressing it a few times; nothing happened and no message. Went into zotero preferences on my laptop standalone. Everything set right. Went into "libraries and feeds"; hard to tell but I think there was no check mark. So made sure there was a checkmark. Went back to sync arrow and clicked; there was encouraging action and I saw a few files upload to zotero.org on library computer. Then I got an error message with this number:

1655211658

and a request to post in forums.

That error was just a temporary server glitch — you can ignore that (and we're currently making changes that should help avoid similar errors).

The linked attachment base directory is not the same as the data directory and definitely shouldn't be set to your data directory. Unless you're using linked files, you should clear that. It's unrelated to the sync error you got.