mount of smb share does work on Debian 11 but not on Debian 12 and uses wrong user

I have a kind of strange bug. I try to mount smb/cifs share of windows server 2019 as server using Debian as client. **The following *exact* command does work on Debian 11.11 but not on 12.9** with myuser as domain user:

mount -v -t cifs "//host/Backup/myhost" /mnt/backup-myhost/  -o "username=myuser,password=mypassword,domain=mydomain"

that gives on both the verbose output

mount.cifs kernel mount options: ip=1.2.3.4,unc=\\host\Backup,user=myuser,domain=mydomain,prefixpath=myhost,pass=*******

Debian 12.9 gives the additional error

mount error(13): Permission denied

I also tried several "sec" and "version" options. The permissions of /mnt/backup-myhost/ is 755 on both. **Windows server says the user "root" (sic!) tries to connect but mount uses "myuser".** Here is the last part of the output of dmesg of Debian 12.9 but I cannot see a cause even after comparing with Debian 11.11: ... [499614.187245] CIFS: fs/smb/client/transport.c: wait_for_free_credits: remove 1 credits total=0 [499614.187248] CIFS: fs/smb/client/transport.c: Sending smb: smb_len=372 [499614.188538] CIFS: fs/smb/client/connect.c: RFC1002 header 0x48 [499614.188542] CIFS: fs/smb/client/smb2misc.c: SMB2 data length 0 offset 0 [499614.188543] CIFS: fs/smb/client/smb2misc.c: SMB2 len 73 [499614.188545] CIFS: fs/smb/client/smb2ops.c: smb2_add_credits: added 1 credits total=1 [499614.188556] CIFS: fs/smb/client/transport.c: cifs_sync_mid_result: cmd=1 mid=2 state=64 [499614.188560] CIFS: Status code returned 0xc0000070 STATUS_INVALID_WORKSTATION [499614.188562] CIFS: fs/smb/client/smb2maperror.c: Mapping SMB2 status code 0xc0000070 to POSIX err -13 [499614.188563] CIFS: fs/smb/client/misc.c: Null buffer passed to cifs_small_buf_release [499614.188566] CIFS: VFS: \\host Send error in SessSetup = -13 [499614.188569] CIFS: fs/smb/client/connect.c: VFS: leaving cifs_get_smb_ses (xid = 111) rc = -13 [499614.188572] CIFS: fs/smb/client/dfs_cache.c: cache_refresh_path: search path: \host\Backup [499614.188576] CIFS: fs/smb/client/dfs_cache.c: get_dfs_referral: get an DFS referral for \host\Backup [499614.188586] CIFS: fs/smb/client/connect.c: VFS: leaving mount_put_conns (xid = 110) rc = 0 [499614.188588] CIFS: VFS: cifs_mount failed w/return code = -13