Out of my league, but an online search suggest the error is due to KDC failing to generate a renewable ticket-granting ticket and one of the solutions consists in increasing the max_renewable_life.
You might try to analyze kinit/klist or the verbose output of the command called by the hourly cronjob (which is trying to refresh TGT for any service with KrbStatus prop set to “enabled”):
/usr/libexec/nethserver/smbads -v tgt
A note about End Of Life of NethServer 6: