PMP450AP 13.1.3 to 14.1 upgrade: CNUT returns "Login authentication failed"

I have met the problem with upgrading PMP450AP from 13.1.3 to 14.1. CNUT 4.12.8 returns the following lines in the log:

07/30/19 07:15:48 INFO Refresh Selected Network Elements process start ..
07/30/19 07:15:48 INFO Refreshing NE at:172.16.2.4
07/30/19 07:15:48 INFO NE at:172.16.2.4 refreshed.
07/30/19 07:15:48 INFO Refreshing SM List: 172.16.2.4
07/30/19 07:15:48 INFO No SMs registered to AP 172.16.2.4
07/30/19 07:15:48 INFO Refresh Selected Network Elements process has completed.
07/30/19 07:16:05 INFO Updating Selected Network Elements process start ..
07/30/19 07:16:05 INFO CANOPY141_2BUILDofficial_PXP45x_S.pkg3 loaded.
07/30/19 07:16:05 INFO Starting Update Process ..
07/30/19 07:16:05 INFO Queue NE:172.16.2.4 for updating
07/30/19 07:16:06 INFO Host: 172.16.2.4;ESN: 0A003EA0EBB4;Message: Checking Network Element Status (SITE= NCAK_W)
07/30/19 07:16:07 INFO Host: 172.16.2.4;ESN: 0A003EA0EBB4;Message: Current = CANOPY 13.1.3 AP-DES, CANOPYBOOT 1.0, 011514 (SITE= NCAK_W)
07/30/19 07:16:07 INFO Host: 172.16.2.4;ESN: 0A003EA0EBB4;Message: Using Package E:\Cambium_Ntwks\PMP450\Releases\14.1.2\CANOPY141_2BUILDofficial_PXP45x_S.pkg3 (SITE= NCAK_W)
07/30/19 07:16:07 INFO Host: 172.16.2.4;ESN: 0A003EA0EBB4;Message: No master/slave register link detected. (SITE= NCAK_W)
07/30/19 07:16:07 INFO Host: 172.16.2.4;ESN: 0A003EA0EBB4;Message: Transferring Files 5x_20_cf200.img to host:172.16.2.4 by http (SITE= NCAK_W)
07/30/19 07:16:28 WARN Host: 172.16.2.4;ESN: 0A003EA0EBB4;Message: Login authentication failed, please verify HTTP(s) credentials on the device(s) are correct. (SITE= NCAK_W)
07/30/19 07:16:28 INFO Node: 172.16.2.4 Update Completed.
07/30/19 07:16:28 INFO Refreshing NE at:172.16.2.4
07/30/19 07:16:28 INFO No SMs registered to AP 172.16.2.4
07/30/19 07:16:28 INFO Updating Selected Network Elements process has completed.

The AP however remains at 13.1.3.

I guessed the problem is that I removed in the past the "root" user (only another with administrator privileges was configured in the far past), hence I added "root" user with "root" password, but it does not help.

Earlier I have upgraded all SMs from 13.1.3 to 14.1, and after that all of them were lost from Sessions, as it seems the 13.1.3 PMP450AP cannot accept PMP450SMs 14.1. I trusted when I upgrade the AP to 14.1, all SMs will return, but I have not expected the trouble with AP upgrade (earlier I have upgraded two other APs 13.1.3->14.1->16.0.1 without problems).

Do I suppose right the problem is with the message:

"Login authentication failed, please verify HTTP(s) credentials on the device(s) are correct" ?

I have been upgrading SMs using directly their IP addresses, not Autoupdate. One SM was not registered before upgrade.

After the ineffective upgrade of this AP, CNUT says in the log that Update is Completed which is very misleading, as 13.1.3 still remains on AP.

I will be grateful for advice what is wrong and how to successfullly upgrade this AP.

Rgds

In the meantime I have found the problem source. The "root" user, which is by default used by CNUT, was removed from the device for security reason, and recreated was with wrong password. PMP450 Configuration and User Guide says nothing on "root" password, unfortunately. Digging I found the default password on AP and SM is "admin" for "root" user with full ADMINSTRATOR privileges. CNUT allows anyway to configure credentials as needed in Edit>Preferences. It solves the problem.