Sorry you’re experiencing issues.
I just checked the stats, and thousands of people have updated to the new version, but you’re the first (maybe only one) to experience this. We will troubleshoot it, but there’s a chance it may not be directly related to Style Pack.
Considering it’s a private site, troubleshooting will be a little more difficult, but let’s start with some basics.
1.) Did you update anything else recently or at the same time as Style Pack?
2.) Do you have moderation tools enabled?
3.) Do you have any words or phrases in the WP Discussion settings for blacklisted/disallowed words.
4.) Do you have debugging enabled and are there any recent entries in the debug log file?
5.) When you make a post, you say it shows up empty/blank. Do the post actually save in the DB? Can you see the empty/blank post in the WP admin panel? Or is it not being saved at all?
6.) Does it only affect the admin user or every user?
7.) Have you gone through the standard troubleshooting procedures of disabling plugins except for Style Pack and bbPress, verifying that works properly, and then re-enabling plugins one at a time, retesting after each one, until you find the conflict?
8.) Have you tried uninstalling v 5.6.0 and rolling back to v 5.5.9 to see if the issue still persists?
There’s other things we can try, but it starts to get complicated quickly. These are the basics to try first.