The GitHub Issuetracker has a workaround:
https://github.com/anegostudios/VintageStory-Issues/issues/407
@PrinceOfStrings workaround is functional
Some packages are missing mainmenu9.png to mainmenu11.png, causing this problem.
Hey, i had the same problem. When i saw your post a quick fix came in mind and the game started again!!
I just copied one of the mainmenu*.pngs and renamed the copies mainmenu9, mainmenu10 and mainmenu11 since the original ones were in fact missing. Started VS and is running just fine!
Peace
just want to add that you can find this folder in your vintagestory installation directory under "assets\game\textures\gui\backgrounds"