Sync icon still spinning
Following the instructions in the documentation, I'm starting a new thread to report that my sync icon has been spinning for many more than two hours with no result. I can successfully sync between my MacBook and the server in just a few moments, and web access to my library shows the correct records. However my desktop computer has not been able to sync at all since I imported a large library of about 10,000 records (but no attached files) several days ago. There may have been a glitch because after importing the library the first time, I noticed several data errors (of my own doing) and had to delete and reimport several times. I am running the standalone version of Zotero (v. 4.0.21.4) on an iMac running 10.9.3.
234K items is a shocking number, and there's no way your current download will finish. But 223K of those items are actually in the trash, so it seems like you reimported quite a few times (and/or have a huge number of child notes and attachments) without emptying the trash after deleting the items.
If you have another computer that's syncing successfully — that is, the computer where you originally added the items and they still exist in the trash — you could delete the items from the trash in, say, 10K-item batches and sync that computer, but even that might be problematic. If you'd like, I can clear your online trash for you, which will probably allow you to sync.