`ipa-server-upgrade` hangs after updating RHEL 7.4 to 7.5

Issue

IPA upgrade stalls at:

[root@ipaserver ~]# ipa-server-upgrade
Upgrading IPA:. Estimated time: 1 minute 30 seconds
  [1/8]: saving configuration
  [2/8]: disabling listeners
  [3/8]: enabling DS global lock
  [4/8]: starting directory server
  [5/8]: updating schema
  [6/8]: upgrading server

And /var/log/ ipaupgrade.log shows the IPA upgrade process stopping at indexing and never finishing:

[root@ipaserver ~]# tail -f ipaupgrade.log
2018-05-09T11:37:14Z DEBUG      nsindex
2018-05-09T11:37:14Z DEBUG nsindextype:
2018-05-09T11:37:14Z DEBUG      eq
2018-05-09T11:37:14Z DEBUG      sub
2018-05-09T11:37:14Z DEBUG cn:
2018-05-09T11:37:14Z DEBUG      nsOsVersion
2018-05-09T11:37:14Z DEBUG nssystemindex:
2018-05-09T11:37:14Z DEBUG      false
2018-05-09T11:37:19Z DEBUG Creating task to index attribute: nsOsVersion
2018-05-09T11:37:19Z DEBUG Task id: cn=indextask_nsOsVersion_137451586397619440_10545,cn=index,cn=tasks,cn=config