There are 2 symptoms found for this issue when Customers are unable to add participants to a Service Management request. 

Symptom 1

1. Click "Add People" in the Service Management request
2. Look up the user that needs to get added as participant
3. Once found, click "Add" to add the participant

After the "Add" button has been clicked, nothing happens. The following appears in the atlassian-jira.log:

http-bio-8080-exec-36 ERROR <user> 646x217666x1 5hs9lj <IP>,<IP> /rest/servicedesk/1/servicedesk/customer/participant/addparticipants/<Issue-ID> [common.error.jersey.ThrowableExceptionMapper] Uncaught exception thrown by REST service: com.atlassian.jira.issue.customfields.impl.FieldValidationException: Boolean parse error: null
com.atlassian.jira.exception.DataAccessException: com.atlassian.jira.issue.customfields.impl.FieldValidationException: Boolean parse error: null

Followed by:

Caused by: com.atlassian.jira.issue.customfields.impl.FieldValidationException: Boolean parse error: null
	at com.xiplink.jira.jql.GitCommitsReferencedCF.getSingularObjectFromString(GitCommitsReferencedCF.java:44)
	at com.xiplink.jira.jql.GitCommitsReferencedCF.getSingularObjectFromString(GitCommitsReferencedCF.java:20)
	at com.atlassian.jira.issue.customfields.impl.CalculatedCFType.getValueFromCustomFieldParams(CalculatedCFType.java:76)
	at com.atlassian.jira.issue.fields.CustomFieldImpl.updateIssue(CustomFieldImpl.java:1264)

 This only appears to happen for Customer user types in Jira Service Management. Agents do not appear to be impacted by this. 

Symptom 2

With the same steps above, a "Failed to update request participants field" error will pop up when trying to add a participant to a request:

With an error below shown in the browser's developer tools console:

{"errors":[{"errorMessage":"Failed to update request participants field,errors:"}],"reasonKey":"com.atlassian.servicedesk.internal.feature.reqparticipants.ParticipantUpdateFailedError","reasonCode":"400"}

Cause

For Symptom 1

This issue is caused by GIT Integration Plugin for Jira version 2.6.9.

For Symptom 2

The issue may be caused by one of the following 3rd-party plugins:

• Rating CustomField/Slie Jira CustomFields
• ServiceRocket Salesforce.com Connector
  • ServiceRocket Core Connector - Plugin (dependency plugin for ServiceRocket Salesforce.com Connector)
  • ServiceRocket Jira Connector - Plugin (dependency plugin for ServiceRocket Salesforce.com Connector)

Workaround

For Symptom 1

Downgrading the add-on to version 2.6.8 will prevent this issue from happening. In addition, version 2.6.10 or above provides a fix as well.

For Symptom 2

Disable those plugins from Manage Add-ons page. Reach out to the plugin's vendor for further investigation.