Our 844 has been in daily use for years now.
As others have said, the vent hole is there to allow steam to escape while the kettle starts to heat and should
never be blocked!
I cannot see how water can get to the vent
There's no reason to be too concerned about the water level as long as it's sufficient for the kettle base to click down and not above the mark.
The most important thing is to see that the seal on the kettle lid is intact. As the kettle heats up, steam escapes from the vent; when it reaches boiling the hole cannot escape sufficiently, so boiling water is poured into the teapot from the spout and when the kettle empties, the base clicks up, light turns on and kettle switches off.
Your Teasmade seems as though it's probably working fine.