Getting UNKNOWN_EXCEPTION while updating SharingCriteriaRule using Metadata API

Question

I am trying to update the access of SharingRule using the Metadata API from APEX. I have created a MetadataService class using https://github.com/financialforcedev/apex-mdapi/blob/master/apex-mdapi/src/classes/MetadataService.cls. I also created a SharingRuleUpdater class with method to update the SharingRule.

public without sharing class SharingRuleUpdater {

  public void updateSharingRule(String sharingRuleName, String accessLevel) {
    // Create the metadata API client
    MetadataService.MetadataPort service = new MetadataService.MetadataPort();
    service.SessionHeader = new MetadataService.SessionHeader_element();
    service.SessionHeader.sessionId = UserInfo.getSessionId();

    
    MetadataService.SharingCriteriaRule scr = new MetadataService.SharingCriteriaRule();
    scr.fullName = sharingRuleName;
    scr.accessLevel = accessLevel;
      
      // Update the sharing rule
    List<MetadataService.SaveResult> result = service.updateMetadata(new MetadataService.Metadata[] {scr});
    System.debug('result-->'+result);
  }
}

After that I have created Shift_Sharing_Rule SharingCriteriaRule on Shift__c custom object. I am executing below code to update the access of this SharingRule

SharingRuleUpdater sru = new SharingRuleUpdater();
sru.updateSharingRule('Shift__c.Shift_Sharing_Rule','Edit');

Executing above code is giving below error

message=An unexpected error occurred. Please include this ErrorId if you contact support:...., message_type_info=(message, http://soap.sforce.com/2006/04/metadata, null, 1, 1, false), statusCode=UNKNOWN_EXCEPTION