Hello everybody,
If I fix the camera on an mesh, then I can not move the mesh (or edges / corners of the mesh) by holding down on the respective axis with the left mouse button.
By the way, where I click with the left mouse button, the crosshair is set. (see screenshot)
But I want to move the objects.
I need your help. Thank you. ^^
Which version of blender are you using? I know there used to be a bug that made it impossible to select the axes of the gizmo under certain circumstances, but I don’t know if it has been fixed, or if i just don’t know how to replicate it.
And by default bleneder uses rightclick select, indended to make the workflow easier, because that way selecting an object and selecting the gizmo to move/scale/rotate the object are separated, so you won’t accidentally move an object if you just want to select something and vice versa. Switch to leftclick select by going to the user preferences, select input at the top, and on the left find the select with setting.
Blender version 2.79.0
I also think that was a bug.
I have moved all the meshes (lights, camera etc) of the file into a new blender file with exactly the same settings (with “Append”). It works now! 
Thank you anyway ^^
EDIT: After a restart of the file the problem occurred again! An update to version 2.79.2 solved the problem.
