If you see a red symbol (with a white cross) near your mousepointer when you try to initialize a vMotion process using Drag & Drop in the vSphere Web Client, do not doubt yourself.
As this is something I expect to work and something I am used to from my work with the vSphere Client, I tried it several times, restarted services, tried it again…
Finally I asked Google and found the following Knowledge Base Article:
It seems to be a known issue with the vSphere Web Client 5.5 – it would be great if VMware can fix this in one of the coming updates.
Of course you can still migrate a virtual machine with your Web Client using the migrate-option.
Just right-click the VM and select “Migrate” from the context menue.