So this is software encryption on the Disk Agent if I see it correctly!? Not sure, but you may want to try:

#  OB2_FORCE_OLD_DECR_MODE=0|1
#    Default value : 0
#    Whenever a session fails to decrypt an encrypted medium, the session fails with this error -
#    [Critical] From: xMA@xyz "HP:Ultrium 4-SCSI_4"  Time: 9/1/2020 12:04:10 PM
#    [90:6100] Drive unable to decrypt data: Invalid decryption key retrieved from key management server.
#    In such a case, set the omnirc variable to 1 on the Media Agent host.
#    Once the session is completed set the omnirc variable back to 0. Also, it is
#    advisable to restart the Data Protector INET on the Media Agent host whenever
#    changes are made to this omnirc variable

I would set this on the DA system. You could also try on CS or MA if it wouldn't work. Besides this I have no idea.

This parameter refers to tape encryption.

In our case, it is Data Protector encryption, it has the option in the job set to AES 256-bit enabled.

We did a test backup and test restore after the upgrade and yes, I can restore normally, only data in the backups from before the upgrade have the problem, it seems that something was lost, is there any other OB2 variable for this type of error. " Received incorrect encryption key. Aborting Disk Agent."

No this means this probably isn't implemented in the DA and as you are on an unsupported version we will never get a fix for it. You may have reasons for upgrading from an unsupported version to another unsupported version, but have to be aware it means no new fixes are possible.

The variable "OB2_FORCE_OLD_DECR_MODE=1" was added and the restore worked successfully.

The client was updated on 07/31. For restores prior to this date, the variable "OB2_FORCE_OLD_DECR_MODE=1" must be set to 1.

For restores after this date, the variable "OB2_FORCE_OLD_DECR_MODE=0" must be set to 0.

We did several tests here and identified this. Is there any other workaround?

Well, using the parameter is the workaround already.