Importing error

[Deleted user]

When we try to import data from Freshdesk, either JSON or CSV, Jira shows the following log:

2020-02-27 14:04:27,407 INFO - Import started by admin using com.atlassian.jira.plugins.importer.sample.SampleDataBean
2020-02-27 14:04:27,422 INFO - ------------------------------
2020-02-27 14:04:27,422 INFO - Importing: Users
2020-02-27 14:04:27,422 INFO - ------------------------------
2020-02-27 14:04:27,422 INFO - Only new items will be imported
2020-02-27 14:04:27,426 INFO - 0 users associated with import. All of them imported as inactive, this can be changed after import in User Access step.
2020-02-27 14:04:27,426 INFO - ------------------------------
2020-02-27 14:04:27,426 INFO - Finished Importing : Users
2020-02-27 14:04:27,426 INFO - ------------------------------
2020-02-27 14:04:27,426 INFO - 0 users successfully created.
2020-02-27 14:04:27,430 INFO - Retrieving projects...
2020-02-27 14:04:27,444 INFO - Project ExternalProject{id='null', key='FAC', externalName='null', name='null', type='null', template='null'} exists and found correctly.
2020-02-27 14:04:27,447 INFO - ------------------------------
2020-02-27 14:04:27,447 INFO - Importing: Versions
2020-02-27 14:04:27,447 INFO - ------------------------------
2020-02-27 14:04:27,447 INFO - Only new items will be imported
2020-02-27 14:04:27,447 INFO - ------------------------------
2020-02-27 14:04:27,447 INFO - Finished Importing : Versions
2020-02-27 14:04:27,447 INFO - ------------------------------
2020-02-27 14:04:27,447 INFO - ------------------------------
2020-02-27 14:04:27,447 INFO - Importing: Components
2020-02-27 14:04:27,447 INFO - ------------------------------
2020-02-27 14:04:27,447 INFO - Only new items will be imported
2020-02-27 14:04:27,447 INFO - ------------------------------
2020-02-27 14:04:27,447 INFO - Finished Importing : Components
2020-02-27 14:04:27,447 INFO - ------------------------------
2020-02-27 14:04:27,453 INFO - Retrieving custom fields...
2020-02-27 14:04:27,459 INFO - ------------------------------
2020-02-27 14:04:27,459 INFO - Importing: Issues
2020-02-27 14:04:27,459 INFO - ------------------------------
2020-02-27 14:04:27,459 INFO - Only new items will be imported
2020-02-27 14:04:27,477 INFO - Importing issue: [externalId='2', summary='TEST']
2020-02-27 14:04:27,531 INFO - ------------------------------
2020-02-27 14:04:27,531 INFO - Finished Importing : Issues
2020-02-27 14:04:27,531 INFO - ------------------------------
2020-02-27 14:04:27,532 ERROR - Unexpected failure occurred. Importer will stop immediately. Data may be in an unstable state: 

The import fails. What could cause this and how can we solve this? The JSON is 18 kb, so the size won't be a problem.

Using Atlassian Service Desk Cloud.

Thanks in advance.