Why do I get the error "Database.update(paymentTxnList)- field integrity exception (Guest users cannot be record owners.)"?
1. Check ALL Payment Forms to see if the Donor Care Person (user) is a Guest User. Go to user record details and check the User Licence. Donor care becomes the owner so that email notification for large donations can be sent.
2. Check ALL Accounts to make sure owner is NOT a force.com sites one. I.e. created prior to the Salesforce release update.
3. To improve data security for orgs with guest users, Salesforce made some security improvements. This is related to Salesforce's spring 22 update: https://help.salesforce.com/s/articleView?id=sf.networks_guest_policies_timelines.htm&type=5