I can hardly only believe that. Did you have a look at them all?

By default, Apache logs in /var/log/apache/, /var/log/httpd/, /var/log/apache2/ or /var/log/httpd2/. There are usually two logfiles, "access_log" and "error_log" (sometimes the underscore "_" is a dot "." or left away). It may well be your Apache logs to logfiles that are split up by your virtual hosts. In rare cases, Apache logs to syslog, what would require an advanced configuration.

OTRS logs to syslog by default, or to /tmp/otrs.log. On Win32, the default is /otrs/var/log/otrs.log, as Win32 doesn't come with a syslogd.

Logging is working correct, i can see all other messages.

Did you try another browser, such as Mozilla/Firebird or Opera?

Same occurs with Mozilla/Firebird.

The behaviour does not occur with small-sized attachments, does it?

Small attachments are working.

Which URL does the browser want to connect to? Does the request, if meant for your OTRS server, reach the server? You may check this with a paket sniffer like Ethereal. Run a copy on both machines, filter on http traffic only. Ethereal is available from http://www.ethereal.com/ (alongside its docs, btw...).

This is my ethereal capture(libpcap), as you can see from paket 111 on something went wrong, but im only a stupid student and dont know what exactly is happening there... :) (See attached file: large_attachment)

Does the problem occur on localhost, too, ie. on the OTRS server itself?

Same problem using Konqueror on localhost.

Do you have some kind of (Desktop) Firewall and/or virus scanner running? Does the behaviour go away when (temporarily) switching those off?

No. mfg Dennis