Weight paint not working (bug?) I have this funny document with two scenes (file attached). The plane in Scene1 is nicely subdivided and set up, but cannot be weight painted. However the plane in Scene2 (which is a straight copy) can be weight painted.  

Can anyone reproduce this behaviour or is this just happening on my end?
When you're weight painting, you're painting into Vertex Groups.

The vertices you can't paint don't belong to  a group.

There's a difference between a vertex belonging to a group with 0 weight, and a vertex not belonging to a group at all.

If a vertex is not assigned to the active group, you can't paint into it.

Assign vertices (with a given weight) to a group in the Data tab > 'Vertex Groups' panel.

It's very confusing, because by default, unassigned vertices are colored blue, like 0-weight vertices.

You can fix that.. in the Overlays dropdown there's an option, even more confusingly called 'Zero Weights'. (IMO, it should be called 'Unassigned Vertices'.) Anyway, you can use it to display vertices unassigned to the active group, or those unassigned to any group, in black, not blue.