SSPI negotiation failed when trying to call the SDK after changing a password

Hi,

We are encountering a security issue when trying to call the SDK (onPremise 8.2) from a web service. This worked fine until we changed to password of the CRM user USER_A which is used as 'Identity' of the appPool under which the  web service is running.

After changing the password, we re-entered the identity as user USER_A + new password and restarted the appPool.Now we are getting the following exception when calling the SDK

SOAP security negotiation with 'http://crm.xxxx.xxx/org1/XRMServices/2011/Organization.svc' for target 'http://crm.xxxxxx.xxx/org1/XRMServices/2011/Organization.svc' failed. See inner exception for more details.
InnerException = {"The Security Support Provider Interface (SSPI) negotiation failed."}

We thought it might be an SPN issue for accessing the SDK service which runs under SVCUSER

>setspn -q http/crm.xxxx.xx
Checking domain DC=xxxxx,DC=org
CN=SVCUSER,OU=Service Accounts,DC=xxxxx,DC=org
        HTTP/crm.xxxx.xxx

We thought we could add USER_A, but this fails (duplicate SPN found) because it already has SVCUSER under which it is running

Any idea why a simple password change would cause this error?
Thanks for your help

Christian