Deleted all certificates by mistake. How to recreate?
Restore the contents of these three directories from a recent backup:
~/Library/Keychains
/Library/Keychains
/System/Library/Keychains
On my system this comprises 18 files and a subdirectory in the user folder ~/Library/Keychains
I finally solved the issue by opening the web sites that misbehaved, one at a time, in Safari. From there you proceed to trust each and every certificate. They are then re-added to your keychain. More can be found in this KB.
I'd like to thank @bmike for doing his best to assist. Restoring from backup would probably work (if I had one - silly me).