BTW, I always restarted Atom after disabling packages. It looked safer to me to do it this way.
I have now reenabled nearly all packages, including the color-tabs package (which I had first suspected to be the culprit), and tabs are still to see - no problem anymore! There are three noticeable exception:
I had originally also the packages directory-color, line-number-color and rainbow-tabs enabled. When I enable them, I get now an error message
Failed to activate the directory-color package
document.getElementsByTagName(…).append is not a function
Show Stack Trace
The error was thrown from the directory-color package. Atom is out of date: 1.16.0 installed; 1.23.3 latest. Upgrading to the latest version may fix this issue.
I am aware that I should update Atom, but I didn’t want to change the current setup while researching the problem.