I'm having issues too. I've added a lot of feeds from an OPML and it gave me an error, so I've deleted all of them and I started to add one by one. I've come to the conclusion that the feeds created from Pubmed are the ones giving problems. Now, my entire library is showing "Loading items..." even though I eliminated all the feeds.
Can you provide a Debug ID for Zotero startup through reproducing this, using the "Restart with Logging Enabled…" option?