When upgrading from SDK 2.5 to 2.7 the chip keeps rebooting with the message <err> zboss_osif: ZBOSS fatal error occured.
After some debugging it turned out to be the zb_nvram_write_data called by migrate_curr_page_to_new_version().
(See also: https://devzone.nordicsemi.com/f/nordic-q-a/113710/zboss_osif-zboss-fatal-error-occurred-when-migrating-sdk-from-2-3-0-to-2-7-0)
No solution to migrate atm. Depending on chosen reset method it is possible to factory-reset the chip (SW1 method does not work because reboot occurs before 5sec timeout). In that case you have to use nrfjprog --eraseall and repair the chip.
Suggestion/Topic to discuss:
Make SW1 factory reset a blocking event, so it can kick before the ZBOSS bootup.