No response while sending attachment

[rtfrex]

Server: hMail Server 5.3.3-B1879 on Windows Server 2008R2
Roundcube 0.8.5 on IIS 7.5 / PHP 5.3.22 / MySQL 5.6

Basically everything working properly.
But just an issue on sending out attachment.

When I attached some files (under the size limit) and click the "Send Message" button,
the status change to "Sending message..." and then no response, but the mail has been sent properly.
I use Chrome Developer Tools Console to monitor, I can see a HTTP 500 Error while sending out the message,
but there are no error in the error log file....

I tried to change the execution time on PHP.ini and also the connection timeout on IIS,
but the problem still persists, please help...

[rtfrex]

or the error like that

[SKaero]

Make sure the logs directory is writeable. If there is a 500 error it will get logged.

[rtfrex]

Make sure the logs directory is writeable. If there is a 500 error it will get logged.

thanks for reply,
yup, it is writable, I have tested to disconnect the database and it can generate errors on the log file,
but no related error about the HTTP 500 issue.... please advise...

[SKaero]

Make sure you have $rcmail_config['log_driver'] set to 'file' in the main.inc.php. if its not showing up in the RoundCube error log you'll need to check the server error logs and see if its showing up there.

[rtfrex]

Make sure you have $rcmail_config['log_driver'] set to 'file' in the main.inc.php. if its not showing up in the RoundCube error log you'll need to check the server error logs and see if its showing up there.

yes, $rcmail_config['log_driver'] set to 'file' already.
Maybe I try to create a custom handler for HTTP 500 Error.... 

[rtfrex]

Make sure you have $rcmail_config['log_driver'] set to 'file' in the main.inc.php. if its not showing up in the RoundCube error log you'll need to check the server error logs and see if its showing up there.

Finally, I found the reason.
The HTTP 500 error is "The FastCGI process exceeded configured activity timeout"
I changed the activity timeout of FastCGI on applicationhost.config setting to bigger value, it works.
Now the problem is when I send out the mail with attach file, it will take a long long long long time.....
but before redirect to Inbox, the mail has been sent already .....