NCBI has announced some testing tomorrow morning from 8 to 9 am:
will be testing https on public web servers from 8:00 to 9:00 AM EDT
(13:00-14:00 UTC) on Thursday, September 15. You may experience problems
with NCBI web sites during that time."
The sites that I expect would have the biggest impact: PubMed, PubMed Central, and the MeSH database.
More information is here: https://www.ncbi.nlm.nih.gov/home/bulletins/https-tests.shtml