Q: Safari Forms AutoFill greyed out

Doubt it's a keychain issue. More likely an extension or plug-in issue.

Try troubleshooting extensions and third party plugins.

From the Safari menu bar click Safari > Preferences then select the Extensions tab. Turn that OFF, quit and relaunch Safari to test. If that helped, turn one extension on then quit and relaunch Safari to test until you find the incompatible extension then click uninstall.

If it's not an extensions issue, try troubleshooting third party plug-ins.

Back to Safari > Preferences. This time select the Security tab. Deselect:  Allow all other plug-ins. Quit and relaunch Safari to test.

If that made a difference, instructions for troubleshooting plugins here.

Ok..

Open the Finder. From the Finder menu bar click Go > Go to Folder

Type or copy paste the following:

~/Library/Caches/com.apple.Safari/Cache.db

Click Go then move the Cache.db file to the Trash.

Quit and relaunch Safari to test.

Sorry to put you through so many hoops but usually happens after an upgrade ...

Might be corrupted Safari preferences ...

Open a Finder window. From the Finder menu bar click Go > Go to Folder

Type or copy paste the following:

~/Library/Preferences/com.apple.Safari.plist

Click Go then move the com.apple.Safari.plist file to the trash.

Quit and relaunch Safari to test.

You could try restoring OS X using OS X Recovery

One more thing...  try deleting login items.

System Preferences > Users & Groups > Login Items

And HD > Library > StartupItems

Then restart your Mac.

If this is indeed a work computer then the reason may be that your IT has installed a profile that disables Safari's password autofill feature. Check in System settings => Profiles => a profile that says "Disable password autocompletion" under the Safari header.