Well, eysteinm, I did what I think you did: I dragged the splitter between the list of messages and the preview pane all the way to the right, and exactly the same thing happened to me: I couldn't drag it back to the left again. The cursor (vertical line with left-pointing arrow) didn't seem to 'grab' the splitter.
After 20 minutes or so of utter frustration I turned to AppleScript.
First, launch System Preferences - Security and Privacy - Privacy, and make sure that Script Editor can control your computer:
Launch Mail and make sure you're in normal, "non-classic" view mode.
Now, launch Script Editor. Copy the following and paste it into a new window:
tell application "System Events"
tell process "Mail"
set {{vert, hori}} to position of splitter of splitter group 1 of splitter group 1 of window 1
set position of splitter of splitter group 1 of splitter group 1 of window 1 to {vert, (hori - 200)}
end tell
end tell
There was no immediate change to the Mail window, but after running this script, I was able to grab the splitter and move it over to the left again, and even after dragging it all the way to the right I could still grab it and drag it back without running the script again.
I can't promise it will work for you, but I hope it helps.
H