this problem had been discussed at least 2 times
Repository with SSL - problem in to do the download - General - F-Droid Forum
Trust anchor for certification path not found - General - F-Droid Forum
using samsung galaxy tab s2 lte with android 7. there are no new updates from samsung. i get this error by adding the newpipe repo.
now i have to rely on f-droid team to deliver updates as soon as possible (that’s why i used the newpipe repo - fast update deliveries)