[Samba] sometimes users fails to login

[Andrea Cucciarre']

Hello,

I have Samba 4.6 as AD domain member and sometime the users fails to 
login, the issue disappear after some minutes.
I have enabled log leve 10 and I can see the following errors:

2019/03/12 09:20:32.280799,  5, pid=15466, effective(0, 0), real(0, 0)] 
../source3/lib/username.c:181(Get_Pwnam_alloc)
   Finding user BITINTRA\U002489
[2019/03/12 09:20:32.281111,  5, pid=15466, effective(0, 0), real(0, 0)] 
../source3/lib/username.c:128(Get_Pwnam_internals)
   Trying _Get_Pwnam(), username as given is BITINTRA\U002489
[2019/03/12 09:20:32.281222,  5, pid=15466, effective(0, 0), real(0, 0)] 
../source3/lib/username.c:153(Get_Pwnam_internals)
   Get_Pwnam_internals didn't find user [BITINTRA\U002489]!
[2019/03/12 09:20:32.282015,  3, pid=15466, effective(0, 0), real(0, 0), 
class=auth] ../source3/auth/user_krb5.c:164(get_user_from_kerberos_info)
   get_user_from_kerberos_info: Username BITINTRA\U002489 is invalid on 
this system
[2019/03/12 09:20:32.282043,  3, pid=15466, effective(0, 0), real(0, 0)] 
../source3/auth/auth_generic.c:145(auth3_generate_session_info_pac)
   auth3_generate_session_info_pac: Failed to map kerberos principal to 
system user (NT_STATUS_LOGON_FAILURE)
[2019/03/12 09:20:32.282196,  3, pid=15466, effective(0, 0), real(0, 0)] 
../source3/smbd/smb2_server.c:3097(smbd_smb2_request_error_ex)
   smbd_smb2_request_error_ex: smbd_smb2_request_error_ex: idx[1] 
status[NT_STATUS_ACCESS_DENIED] || at ../source3/smbd/smb2_sesssetup.c:134

my understanding of the code is that getpwnam fails, which is supposed 
to query winbindd.
In the log file log.wb-BITINTRA I can see the following error:

[2019/03/12 09:20:24.540456, 10, pid=15439, effective(0, 0), real(0, 0), 
class=winbind] ../source3/winbindd/winbindd_cm.c:1014(cm_prepare_connection)
   cm_prepare_connection: connecting to DC WG101SC0002.BITIntra.de for 
domain BITINTRA
[2019/03/12 09:21:04.540067,  5, pid=15439, effective(0, 0), real(0, 0)] 
../lib/tdb_wrap/tdb_wrap.c:64(tdb_wrap_log)
   tdb(/opt/samba/var/lock/mutex.tdb): tdb_brlock failed (fd=22) at 
offset 592 rw_type=2 flags=1 len=1
[2019/03/12 09:21:04.540189,  1, pid=15439, effective(0, 0), real(0, 0)] 
../lib/tdb_wrap/tdb_wrap.c:64(tdb_wrap_log)
   tdb(/opt/samba/var/lock/mutex.tdb): tdb_lock failed on list 106 
ltype=2 (Interrupted system call)
[2019/03/12 09:21:04.540219,  0, pid=15439, effective(0, 0), real(0, 0)] 
../source3/lib/util_tdb.c:497(tdb_chainlock_with_timeout_internal)
   tdb_chainlock_with_timeout_internal: alarm (40) timed out for key 
WG101SC0002.BITIntra.de in tdb /opt/samba/var/lock/mutex.tdb
[2019/03/12 09:21:04.540384,  1, pid=15439, effective(0, 0), real(0, 0)] 
../source3/lib/server_mutex.c:97(grab_named_mutex)
   Could not get the lock for WG101SC0002.BITIntra.de
[2019/03/12 09:21:04.540508,  0, pid=15439, effective(0, 0), real(0, 0), 
class=winbind] ../source3/winbindd/winbindd_cm.c:1023(cm_prepare_connection)
   cm_prepare_connection: mutex grab failed for WG101SC0002.BITIntra.de
[2019/03/12 09:21:04.540667,  1, pid=15439, effective(0, 0), real(0, 0), 
class=winbind] ../source3/winbindd/winbindd_cm.c:1320(cm_prepare_connection)
   Failed to prepare SMB connection to WG101SC0002.BITIntra.de: 
NT_STATUS_POSSIBLE_DEADLOCK

my understanding is that it was hanging locking an offset in the file 
/opt/samba/var/lock/mutex.tdb, so when the timeout elapsed the process 
was interrupted (I guess the offset was that of the mutex for 
WG101SC0002.BITIntra.de)
Could it be a corrupted mutex.tdb file? A slow responding DC?
Any other suggestion?

Thanks
Andrea