KRB_AP_ERR_MODIFIED is most of the time caused by a SPN that is not set on the correct AD Account.
From your DC or any Windows 2016+, you can run the following command:
setspn -Q CIFS/Share
setspn -Q CIFS/Share.yourdomain.com
This will query your AD Database to see if this SPN exist and on which account it is currently set.
Please note that CIFS/Share may not be set and it is not required because the SPN HOST/Share also work for this.
To add the CIFS/Share SPN on the Server1 account, run the following command:
setspn -S CIFS/share Server1
setspn -S CIFS/share.yourdomain.com Server1
The reason why i suggest you to add the FDQN as well as the short name is because you don't know how the client could connect to your server.