Can you provide a Debug ID for Zotero startup through a sync that produces this error, using the Help → Debug Output Logging → "Restart with Logging Enabled…" option?
How long has this been happening? The download — or the post-download processing — is timing out after 30 seconds, despite it being a small (<2 MB) PDF file. It's possible this is due to security software on your system.
D320319660. Thanks
If you're running any non-standard security software, you should check its logs.
I tried logging in to the Zotero website, hoping it would recognize me again and let me continue. That didn't work. Any other ideas?