UPDATE 1:

Testing of the production application indicates the V16 calling server can become unresponsive in a few minutes when the failing remote server connection uses DRIVER=Adaptive Server Anywhere 9.0 or earlier, but it will run for hours without problems when DRIVER=SQL Anywhere 10 or later (connect after 5 seconds, timeout after 15 seconds, drop, repeat, looping through 30 remote servers).

Production tests have been run using V7, V8, V9, V10 and V16 DRIVER values.

Work is now proceeding on a simple reproducible. Motivation: I can ignore this somewhat obscure problem with V5.5 targets... but not Version 9! ...that version is still in heavy rotation out here in RealVille! :

The specific question is, "Why does dbsrv16 build 1915 crash in dbl50t.dll when using SERVER class SAODBC?"

But it's not just V5.5...

During testing of multiple failing (database not found, etc) server class SAODBC connections to V5.5 through V16 remote databases, the calling dbsrv16 server on Windows 7 frequently becomes unresponsive or shuts down altogether with NO diagnostics, with the following single exception: a Windows 7 event (see also the attached *.evtx file).

The unresponsive/shut down symptom ALSO occurs when V5.5 is NOT involved, this just happens to be the only diagnostic available (no console log messages, no dumps, nothing)...

Log Name:      Application
Source:        Application Error
Date:          8/21/2014 7:28:29 PM
Event ID:      1000
Task Category: (100)
Level:         Error
Keywords:      Classic
User:          N/A
Computer:      Envy
Description:
Faulting application name: dbsrv16.exe, version: 16.0.0.1915, time stamp: 0x5373e071
Faulting module name: dbl50t.dll, version: 5.5.5.2787, time stamp: 0x3a6a5407
Exception code: 0xc0000005
Fault offset: 0x0000c768
Faulting process id: 0xfdc
Faulting application start time: 0x01cfbd93d0557b48
Faulting application path: C:\Program Files\SQL Anywhere 16\Bin32\dbsrv16.exe
Faulting module path: C:\Program Files (x86)\Sybase\SQL Anywhere 5.0\win32\dbl50t.dll
Report Id: db1e5fb8-298a-11e4-97e6-c80aa9dfe9d8
Event Xml:
<Event xmlns="http://schemas.microsoft.com/win/2004/08/events/event">
  <System>
    <Provider Name="Application Error" />
    <EventID Qualifiers="0">1000</EventID>
    <Level>2</Level>
    <Task>100</Task>
    <Keywords>0x80000000000000</Keywords>
    <TimeCreated SystemTime="2014-08-21T23:28:29.000000000Z" />
    <EventRecordID>259929</EventRecordID>
    <Channel>Application</Channel>
    <Computer>Envy</Computer>
    <Security />
  </System>
  <EventData>
    <Data>dbsrv16.exe</Data>
    <Data>16.0.0.1915</Data>
    <Data>5373e071</Data>
    <Data>dbl50t.dll</Data>
    <Data>5.5.5.2787</Data>
    <Data>3a6a5407</Data>
    <Data>c0000005</Data>
    <Data>0000c768</Data>
    <Data>fdc</Data>
    <Data>01cfbd93d0557b48</Data>
    <Data>C:\Program Files\SQL Anywhere 16\Bin32\dbsrv16.exe</Data>
    <Data>C:\Program Files (x86)\Sybase\SQL Anywhere 5.0\win32\dbl50t.dll</Data>
    <Data>db1e5fb8-298a-11e4-97e6-c80aa9dfe9d8</Data>
  </EventData>
</Event>