I had an error in my lua code in line 24.
I had written "-_" instead of "--" to start a commented line.
-> To better see those errors ingame install the addon "BugEater". It will show the error messages in the chat and you can configure them to only show once etc. Need to visit the settings of this addon in the menu.
Just in case you copied the source code BEFORE I changed this just copy the code again.
Here you'll find a working version so you can check the difference with yours:
No Actionbar Borders v0.0.1