I was thinking about this bug after I went to bed. :^)
Every time Zim starts (in Windows), it opens %USERPROFILE%/.config/zim/zim.log for writing errors (this is a change from 0.48 where it would try to write a file in %PROGRAMFILES%, which is BAD). Unfortunately I forgot to add a line of code to create this folder if it doesn't exist -- and that happens if you've never run Zim!
Restating the workaround so it should for for anyone, do this in the Windows Command Prompt:
Then try launching Zim again.
I'll fix this and redo the release by the end of this week.