Yes, "send failed to clilent" means it failed to send to your application, not to IQConnect.exe itself.
There isn't any documentation that further explains the IQConnectLog.txt file, but most of it is straightforward. The important thing is to turn on the right details for the kind of problem you are trying to solve. On the Logging tab of the Diagnostics app, you can select a combation of Level 1, Level 2, and Lookup information as appropriate to what process(es) you want to make a log of. Within each of those groups, "Requests" will log your requests to the API; "Data" will log the API's responses. These will appear in "FROM CLIENT" or "TO CLIENT" messages, respectively. Like this:
FROM CLIENT LookupRequest 6312 16 2020-12-23 17:42:50 HIT,QNG#C,60,20201223 105200,20201223 235959,,,,1
This is a historical API lookup command sent by the user. A response would look like:
TO CLIENT LookupData 6312 16 2020-12-23 17:42:50 2020-12-23 10:53:00,2.698,2.695,2.695,2.695,36869,34,
TO CLIENT LookupRequest 6312 16 2020-12-23 17:42:50 !ENDMSG!,
TO CLIENT LookupData 6312 16 2020-12-23 17:42:50 !ENDMSG!,
This is good for troubleshooting situations where you are getting unexpected results.
"Error" will include any error messages, and it's advisable to include that if you are troubleshooting.
Searching the log file for "error" will turn up any errors that IQFeed detected. A typical one is:
TO CLIENT LookupError 3896 32 2020-12-23 17:54:07 E,Socket Error: 10038 (WSAENOTSOCK).,
The page https://docs.microsoft.com/en-us/windows/win32/winsock/windows-sockets-error-codes-2
lists all these socket error codes and what they mean. "WSAENOTSOCK" is a common one that means "a socket operation was attempted on something that is not a socket." Which in turn means that the socket wasn't active or wasn't initialized correctly.
"Debug" usually isn't necessary for troubleshooting purposes, unless directed by support. "System" includes internal system messages, which are often helpful but very detailed.
You don't want to log everything willy-nilly, because the file can become very large, and it can consume system resources. It is wise to only enable logging when you are tracking a problem. IQFeed support can also advise you on which elements to include, and can review a log for you.
I hope this is helpful! Let me know if you have any other questions!
DTN IQFeed Implementation Support Specialist