Ok, here is essentially v6.2.1 with debugging turned on, the .pdb file for debugging yourself, and the .map file (warning, looking up an address may or may not reveal the correct function since static functions aren't included).
The big difference here is when the program crashes the handler to write the error to c:\ioftpd.crash.log isn't installed. This means you get the regular crash box window popup. If you click on don't send (if you can see it) or cancel it will leave a small dump of the memory of the application and some configuration information you can send to me that I can debug with.
For XP (not sure about 2003 or Vista) without VS2005 installed the dump will be:
C:\Documents and Settings\All Users\Application Data\Microsoft\Dr Watson\drwtsn32.log and user.dmp
The .log file will contain summary info on the last 10 applications that crashed. Before sending this to me go ahead and remove the first 9 if you want since I won't need them. You can configure the log to just contain the most recent crash by running "c:\windows\system32\drwtsn32.exe" on XP. Vista I think uses a newer tool but I think it might honor the same settings...
For systems with VS2005 (VS2003?) installed things are a little different. There are two temporary files (randomly generated) created in C:\Documents and Settings\username\Local Settings\Temp. random_appcompat.txt and random.dmp. This is your only chance to copy the system configuration/summary information from the random_appcompat.txt file so if you can make a copy. If you hit cancel both files are erased. You can't just copy the .dmp file as it's locked. You can unlock the .dmp file (via a tool like unlocker) but it's easier to just use the debug button from the popup, then once VS2005 loads use Debug/Save dump as... and save the dump to a file.
Here's the link to the executable/debug info only. Just drop it in the system dir and start it manually since ioFTPD-Start won't start the -debug executable. Running this as a service under the Local Service account is a bad idea since I think it runs into permission issues writing the dump files. You also might have issues running under Limited User accounts under XP as they might not be able to create the .dmp file either but I haven't tried that yet.
For testing purposes there is a new site command "site crashnow" which is hard coded to require the M flag for obvious reasons. It will however allow you to play with crashing the server, or in the future if you start seeing really odd behavior to crash it and generate a dump.
In the future I might provide a nice custom crash popup to automate some of this though...
ioFTPD-v6.2.2-debug-only.zip
If you get crash data send me a PM and I'll give you an ftp address where you can upload it.