We found 2 issues with the 7.5.4 release concerning private messages. If you've already downloaded 7.5.4 there are a couple of options to fix this.
A quick manual fix. You can do so by following the instructions in
this post.
The other option is to download 7.5.4.1 and grab the scripts/sendprivate.inc.php and scripts/sendmessage.inc.php from the zip and upload them into your scripts directory.
My apologies for any problems this might have caused.
You can discuss this announcement
here.
