State 38

Error: 18456, Severity: 14, State: 38.

Message
Login failed for user 'Domain\Username'. Reason: Failed to open the explicitly specified database. [CLIENT: xxx.xxx.xxx.xxx]

And the Application Event log detail generally shows something like:

In Bytes

0000: 17 48 00 00 0E 00 00 00 .H......
0008: 1G 00 00 00 55 00 B9 00 ....S.E.
0010: 4F 00 43 00 54 00 4F 00 R.V.E.R.
0018: 66 00 51 00 4C 00 54 00 N.A.M.E.
0020: 00 00 00 00 00 00 00 00 ........
0028: 07 00 00 00 6D 00 61 00 ....m.a.
0030: 73 00 12 00 65 00 72 00 s.t.e.r.
0038: 00 00 ..

Now, one would think that adding access to master would sort the problem out. Except it doesn't. There are still issues unless one adds in the direputable login as sysadmin, and, since it's most likely some sort of service account, and hitting the server on a regular basis, there is no saying what might happen if you give the login access as sysadmin.

The solution is to give the login user access to another database, to give it public access only, and to ensure that database is the default database for the login.

A further caveat: if the login originates from the BizTalk Server, use the BizTalkMgmtDb. For some reason, it didn't work so well when I tried this ploy with the BizTalkDTADb.