I had a very similar issue these days after iOS 14.2 update on my (old) iPhone SE. Initially a hard reset seemed to work (useful instructions for that can be found on the iPhone wiki page), but when I charged my apps again through my back-up, the same problem appeared again. I noticed a VPN server app was the problem, and deleting this app the whole issue was solved and my iPhone works perfectly again. To me it seems to be a compatibility problem between the updated iOS and in my case a VPN server app. I hope this might help for some of you; a simple sollution, but hard to find and very frustrating...