@Vesku I have also never experienced any problems with the Panasonic 100-300mm, and I use it a lot. I'm sorry about your troubles.
When I look inside the lens I see the moving AF lens parking itself to the camera end of objective when powering off. When again powering on the moving lens it not coming to working position in the middle of the objective, it just sticks its off position with a little AF motor jamming sound. It tries to initialize the AF but something sticks.
When I point the camera/lens down and shake a little during power on the moving lens comes out of its "nest". Gravity and shaking helps.
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