I had a locked viewport while I was using the “Blend surface” tool. At some point of adjusting the handles I wanted to rotate the camera, but Rhino 7 seems to forbid unlocking the viewport while the “Blend surface” tool is still active.
The same goes when the viewport is not locked, but I want to lock it while executing “Blend surface”. It’s basically not possible to lock the viewport during that time. 
Could it be the new viewport behaviour?
If you turn the viewport with CNTRL+SHIFT it snaps to ortho views and locks.
Maybe is this?
No, the “Locked” option that appears on the dropdown menu upon clicking on the viewport title simpyl refuses do work while “Blend surface” is active.
Just a reminder that this is still not resolved…