Activate addon: addon not found

I created an addon and packed it as a zip file (it has 3 python files:, and
When I try to install it, it works fine but when I click on the little box to activate the addon, nothing happens. Only "addon not found: ‘test_addon’ " shows up in the console.
Howevery when I put the three python files into one python script, the activation works fine.
Does anyone knows a fix?

EDIT: I uploaded the addon as a python script and as a zip file (2.7 KB) (7.9 KB)