Render to texture with disabled camera allways generates error

When I render to texture with an enabled camera, there are no errors.

But I don't want to render to texture every frame, so I disabled the camera and I use

 Camera.Render();

from an Update(); function just when I need it. But every time I do this, I get this error:

GUI Window tries to begin rendering while something else has not finished rendering! Either you have a recursive OnGUI rendering, or previous OnGUI did not clean up properly.

If I remove the Camera.Render(); there is no error and if I enable the camera game object so that it renders every frame, there is no error.

Does anyone know a solution?