Certificate problems with Safari

I'm having troubles with certain secure websites that use certificates (just a few sites, not all). Websites that didn't cause any troubles before, all of a sudden were asking permission to use their certificates. When I gave this permission (I trust these sites, just normal websites), it seemed to work like before. But after some time, they just didn't work anymore. When I try to load these pages, Safari gives the message that no safe connection can be established. I tried to 'reset' Safari (just by deleting cache, keychains and other files, since there is no option for this in the settings anymore after the update to Sierra), and after that it seemed to work again (that means, I still had to give permission to use some certificates), but not for long. This is really annoying. I hope an update of Safari will arrive soon which fixes this problem, but if not, I hope there is another solution for this. Can anyone tell me what could be the problem?