Left crank arm of the Neo Smartbike no longer holds

Hello everyone,

After about a year, the left crank on my Neo Smartbike has come loose. I have tightened it several times but it loosens again after a few turns. All I have to do is cycle normally. No getting out of the saddle, no extreme strain from sprints.

The solution on the net can't be the last word in wisdom.

www.youtube.com/watch

Is there another solution?

Thanks for your help and best wishes

Torsten