Here is what worked for me for the same issue on my LG G4 after Marshmallow update after hours of trial and error:
1. Make sure you are on WiFi
2. Put the phone on Airplane mode
3. Disable/Unsync Google contacts under "Settings->Accounts->Google-> Contacts Sync"
4. inside the Account and Sync settings, remove all accounts, like Google, Facebook, LG, Gmail, Skype
5. Clear Cache and Data of Contacts under "Settings->General->Apps->Contacts->storage"
6. Clear Cache and Data of Gmail under "Settings->General->Apps->Gmail->storage"
7. Clear Cache and Data of Google under "Settings->General->Apps->Google->storage"
8. In the same way go through each app in the list and clear cache and also data if the data for the that app is something you can get rid of
9. Turn off Airplane mode
10. Add back Google account under Settings->General->Account and Sync
11. Enable /Sync Google Contacts under "Settings->Accounts->Google-> Contacts Sync"
12. Now your Contact Apps will have all the local and Google/Gmail contacts