I was about to say that the post failed because of the attachment size, but noticed that I had previously set the limits higher at:
Max number of attachments per post: 5
Max size of attachment: 300k
Max total per post: 1650k
...so your 297k won't have failed because of that.
There is an ongoing problem with an interaction on my host which has to do with PHP release levels. This causes the white screen problem. SMF want me to upgrade to the latest version of PHP but I can't do that as it will screw all the other users of the host...
Without passing this to an (expensive) webserver techie, I can't really see any way out of it at the moment.