Unable to select a vertex by clicking I can select vertices by using "b" for Box Select, but I'm unable to click a single vertex by a right-click or left-click.

When I right-click a vertex, the mouse pointer switches to another location instead.

When I left-click a vertex, nothing happens at all.

UV-Sculpt is disabled.

I wanted to select a single vertex in order to select an edge loop.

The mesh has been created by adding a cylinder, then assign "Nothing" and then using the Solidify modifier.

Thank you.
I'm not sure what exactely was wrong.
I clicked "Load Factory Settings", and it worked again.