Hi Morten,
The workaround that we added into 4.5.3.2 beta 6 should have fixed the crash for you, because you had PM set to review the email(s) before sending. I tried to recreate the problem using the exact settings that you show in the attachment, but it worked fine for me. If you're reviewing the email before it sends, then the beta 6 build should be working for you as well.
We submitted a bug report with Apple and provided them with an AppleScript that causes Mail to crash on OS 10.5.3 and 10.5.4. Hopefully, this problem with sending multiple attachments will be resolved shortly.
Regards,
Jerry