samba - Mar 2013 - Upgrade from 3.5.6 to 3.6.12 causes errors in password TDB

We're having trouble with our samba PDC since the upgrade to 3.6.12. 
We've
got a standard smbpassword file using TDB and I;ve run tdbtool and
tdbbackup over the file and both report no errors.

The PDC will run for several hours handling hundreds of users and will
then catastrophically fail with each daemon process reporting the following
in turn:

==============================================================[2013/03/05
08:46:09.378281,  0] lib/util.c:1117(smb_panic)
  PANIC (pid 15900): internal error
[2013/03/05 08:46:09.387147,  0] lib/util.c:1221(log_stack_trace)
  BACKTRACE: 39 stack frames:
   #0 /usr/local/samba/sbin/smbd(log_stack_trace+0x1a) [0x2af15816e065]
   #1 /usr/local/samba/sbin/smbd(smb_panic+0x55) [0x2af15816e169]
   #2 /usr/local/samba/sbin/smbd [0x2af15815f72c]
   #3 /lib64/libc.so.6 [0x2af15a39f2f0]
   #4 /usr/local/samba/sbin/smbd(tcopy_passwd+0x27) [0x2af15814dde7]
   #5 /usr/local/samba/sbin/smbd(pdb_copy_sam_account+0x94) [0x2af15812d607]
   #6 /usr/local/samba/sbin/smbd(pdb_getsampwsid+0x188) [0x2af158131666]
   #7 /usr/local/samba/sbin/smbd(_samr_OpenUser+0x10b) [0x2af1580738f3]
   #8 /usr/local/samba/sbin/smbd [0x2af15808197e]
   #9 /usr/local/samba/sbin/smbd [0x2af158090ee4]
   #10 /usr/local/samba/sbin/smbd(dcerpc_binding_handle_raw_call_send+0xba)
[0x2af1581d49a1]
   #11 /usr/local/samba/sbin/smbd(dcerpc_binding_handle_call_send+0x28e)
[0x2af1581d4c90]
   #12 /usr/local/samba/sbin/smbd(dcerpc_binding_handle_call+0x96)
[0x2af1581d4d87]
   #13 /usr/local/samba/sbin/smbd(dcerpc_samr_OpenUser_r+0x20)
[0x2af1580fb88a]
   #14 /usr/local/samba/sbin/smbd(dcerpc_samr_OpenUser+0x1d)
[0x2af1580fb8ac]
   #15 /usr/local/samba/sbin/smbd [0x2af15802b909]
   #16 /usr/local/samba/sbin/smbd(_netr_ServerAuthenticate3+0x297)
[0x2af15802c3a2]
   #17 /usr/local/samba/sbin/smbd(_netr_ServerAuthenticate2+0x5d)
[0x2af15802c90c]
   #18 /usr/local/samba/sbin/smbd [0x2af1580325de]
   #19 /usr/local/samba/sbin/smbd [0x2af15808dc42]
   #20 /usr/local/samba/sbin/smbd(process_complete_pdu+0x264)
[0x2af15808e21a]
   #21 /usr/local/samba/sbin/smbd(process_incoming_data+0x3c4)
[0x2af15809014c]
   #22 /usr/local/samba/sbin/smbd(np_write_send+0x166) [0x2af15809033f]
   #23 /usr/local/samba/sbin/smbd [0x2af157ea0f87]
   #24 /usr/local/samba/sbin/smbd [0x2af157ea1518]
   #25 /usr/local/samba/sbin/smbd(reply_trans+0x6e4) [0x2af157ea2186]
   #26 /usr/local/samba/sbin/smbd [0x2af157f01097]
   #27 /usr/local/samba/sbin/smbd [0x2af157f05296]
   #28 /usr/local/samba/sbin/smbd [0x2af157f05567]
   #29 /usr/local/samba/sbin/smbd [0x2af157f055d8]
   #30 /usr/local/samba/sbin/smbd(run_events_poll+0x3c9) [0x2af15817c09a]
   #31 /usr/local/samba/sbin/smbd(smbd_process+0xa0b) [0x2af157f042be]
   #32 /usr/local/samba/sbin/smbd [0x2af1583c182f]
   #33 /usr/local/samba/sbin/smbd(run_events_poll+0x3c9) [0x2af15817c09a]
   #34 /usr/local/samba/sbin/smbd [0x2af15817c4cf]
   #35 /usr/local/samba/sbin/smbd(_tevent_loop_once+0x84) [0x2af15817c7e9]
   #36 /usr/local/samba/sbin/smbd(main+0x13bc) [0x2af1583c2f6c]
   #37 /lib64/libc.so.6(__libc_start_main+0xf4) [0x2af15a38c994]
   #38 /usr/local/samba/sbin/smbd [0x2af157e88ee9]


Is this a known issue?


-- 
Jonathan Knight
IT Services
Keele University