Constant popups for bla-bla wants access to bla-bla keychain
https://discussions.apple.com/thread/5680074?tstart=0

I got this from an answer on the Apple.com site link (above) after about 6 hours searching and trying different solutions.  It's the only one that hasn't shown at least one of those login prompts in the first 5 minutes.  Thanks to 'rccatalyst' for providing it.  I'm copying it here, because Apple support just hasn't worked for me. The thread linked above it 4 years old and yet it hasn't turned up in any of my FAQ/google/KB searches.

Open 'Terminal' and run the commands below.

cd ~/Library/Keychains

ls

(When you 'ls' here you will see a folder that contains a bunch of random number like so:
94ED610F-DD96-4ECF-A2BC-7D2F8653A464 (this number is unique on each system))

rm -rf 94ED610F-DD96-4ECF-A2BC-7D2F8653A464 (i.e. whatever your code is)

Now reboot your Mac.

When you log back in everything should be back to normal.