[kubuntu] Win7 Virtualbox refuses to boot after updating to trusty

Hi,

I recently upgraded from kubuntu 12.04 to 14.04 (64Bit).
Since then my Windows7 prof 32bit virtualbox (4.3.10) inside kubuntu is not working anymore giving

error. After a few hours of searching the web I ended up here.

What I tried so far:

1) Reinstall virtualbox including all packages

2) Reboot Win7 from disk and try to recover the bootmgr by:

without any success. Does anyone have a hint how to repair the Virtualbox?

Thanks for your help,

Martin