Kmail Asks for Kwallet Password After Every Reboot

I'm using Kubuntu and I have just upgraded from KDE 5.5.5 to 5.6.5. However, after the upgrade, Kmail asks for the Kwallet password after every reboot.

I have tried to re-coect Kmail via Kwallet Manger or reset Kwallet by removing ~/.local/share/kwalletd and ~/.kde/share/apps/kwallet but they didn't work. How can I get rid of the prompt? Thanks in advance. :)