Notes

Kerberos is case-sensitive in many places and the error messages (or lack thereof) may not be sufficiently explanatory. Check the case sensitivity of the service principal in your configuration files. The convention is HTTP/fully.qualified.domain@REALM.
Browsers have varying levels of restriction when dealing with SPNEGO negotiations. Some will provide the local Kerberos ticket to any domain that requests it, while others whitelist the trusted domains. See https://docs.spring.io/autorepo/docs/spring-security-kerberos/1.0.2.BUILD-SNAPSHOT/reference/htmlsingle/#browserspnegoconfig for common browsers.
Some browsers (legacy IE) do not support recent encryption algorithms such as AES, and are restricted to legacy algorithms (DES). This should be noted when generating keytabs.
The KDC must be configured and a service principal defined for NiFi and a keytab exported. Comprehensive instructions for Kerberos server configuration and administration are beyond the scope of this document (see https://web.mit.edu/kerberos/krb5-current/doc/admin/index.html), but an example is below.
Kerberos tickets may use AES encryption with keys up to 256-bits in length, and therefore unlimited strength encryption policies may be required for the Jave Runtime Environment (JRE) used for NiFi Registry when Kerberos SPNEGO is configured.

Adding a service principal for a server at nifi.nifi.apache.org and exporting the keytab from the KDC:

root@kdc:/etc/krb5kdc# kadmin.local
Authenticating as principal admin/admin@NIFI.APACHE.ORG with password.
kadmin.local:  listprincs
K/M@NIFI.APACHE.ORG
admin/admin@NIFI.APACHE.ORG
...
kadmin.local:  addprinc -randkey HTTP/nifi.nifi.apache.org
WARNING: no policy specified for HTTP/nifi.nifi.apache.org@NIFI.APACHE.ORG; defaulting to no policy
Principal "HTTP/nifi.nifi.apache.org@NIFI.APACHE.ORG" created.
kadmin.local:  ktadd -k /http-nifi.keytab HTTP/nifi.nifi.apache.org
Entry for principal HTTP/nifi.nifi.apache.org with kvno 2, encryption type des3-cbc-sha1 added to keytab WRFILE:/http-nifi.keytab.
Entry for principal HTTP/nifi.nifi.apache.org with kvno 2, encryption type des-cbc-crc added to keytab WRFILE:/http-nifi.keytab.
kadmin.local:  listprincs
HTTP/nifi.nifi.apache.org@NIFI.APACHE.ORG
K/M@NIFI.APACHE.ORG
admin/admin@NIFI.APACHE.ORG
...
kadmin.local: q
root@kdc:~# ll /http*
-rw------- 1 root root 162 Mar 14 21:43 /http-nifi.keytab
root@kdc:~#