For the past 4 days the ScienceDirect site itself has been intermittently having problems. I think that the issue is a tracking script that times out. A test of the problem outside Zotero found the same issue. Going to a ScienceDirect record and attempting to download a pdf (outside of Zotero) causes lenghy freezes of Firefox or Chrome. That may not be the root of your problem but it could be a contributer. It certainly has been trying my patience. Everything works fine if I connect at the right moment with or without Zotero. Then, after a few minutes of productive work, the problem returns.

The issue was mentioned on a listserv for reference librarians. One post said that Elsevier offices seem to be closed this week.

Thanks for the hint! For some reason Zotero stopped auto-updating. Manually updated to 4.0.24.1 and sciencedirect works again.