1 - After updating to 4.15 the actual content of my entries has vanished in the edit entry screen. The excerpt and title remain in the recent posts screen at the dashboard.
2 - Also at the dashboard I receive the following error: Movable Type was unable to write to its 'support' directory. Please create a directory at this location: /home/transmag/public_html/mt-static/support/dashboard/stats/0/000/002, and assign permissions that will allow the web server write access to it.
I have verified that this folder does exist, and it has full write permissions.
3 - Loading up design templates generates the following error: Can't call method "classlabelplural" on an undefined value at lib/MT/Template/ContextHandlers.pm line 911.

I had a similar problem with 4.1. When editing an entry under the blog the content was gone. However, when I switched to System Overview|Manage|Entries the content reappeared. I've since moved my account to a different host environment and no longer have the problem. Not sure why it was a problem to begin with.
Forgot to add that after I updated the entry through the system overview/manage/entries the content reappeared under the blog setting for manage\entries.
I have seen this problem too recently on a client's site. I am unsure as to the root cause and cannot reproduce reliably. The problem just went away.
Must be Voodoo.
I ended up nuking the entire install, going back to an old 4.0 database backup and reinstalling v4.1 (non beta). All seems to be ok now, I was a tad nervous the same thing would happen. It's too bad, as for the while I was fiddling with 4.15 it was MUCH faster than 4.1. Hopefully they get the issues sorted out quickly.