Question:

Duplicate Check doesn't work anymore. For example, DC Live does not show any results on a record, DC Job gives no results, and when I search for a record in the Salesforce search bar, nothing shows up.

Answer:

This appears to be related to Salesforce rolling out the Einstein Search feature. You can confirm that your problem is related to this by going to Salesforce Setup > Einstein > Einstein Search > Search Manager. If the Storage Size there states '0B', and if you get errors when clicking 'Show', this is indeed the case.

There is a workaround that will partially solve this: add the Objects you use most often in DC to a number of profiles in the Einstein Search settings. There is a maximum of 5 Objects that can be added per profile. 

Note that it will take a while before Duplicate Check is working again for the added Objects, generally between 30 minutes to a couple of hours.

Another workaround could be to disable a few Einstein Search settings, but this does not always have the desired effect.

Both workarounds are explained below.

Workaround 1:

Add Objects to always search to profiles (available for max 5 Objects).

1. In the Salesforce Setup, go to Einstein > Einstein Search > Objects to Always Search.
2. At the Default Search Profile, click Edit.
3. Move the 5 Objects you use most often in Duplicate Check to the Selected Objects column at the right.
4. Click Save.
5. Repeat for all profiles that make use of Duplicate Check.
6. Wait a while (30 minutes to a couple of hours) until DC is working again for the added Objects.

 Workaround 2:

Disable Einstein Search settings (is not always effective).

1. In the Salesforce Setup, go to Einstein > Einstein Search > Settings
   
2. Untick the settings Personalization, Actionable Instant Results, and Natural Language Search.
3. Click Save.