WL could not be re-opened - the process WaveLabPro9_5.exe kept on running after exiting and had to be killed manually. So I did some testing and found out this:
if plugin in Master section is locked (right-click checkbox Lock) WL does not exit properly and needs to be killed manually.
Not sure if ALL plugins are doing it but for example these definitely are:
Brickwall Limiter
MBit+ Dithering
MasterRig
Voxengo SPAN
And for example these are OK:
Peak Master
Leveler