I have just published a new release.
Though, it may still conflict with the CKEditor plugin at phplist, it adds comments around the injected code to make clear where it comes from.
I'll try to learn more about the CKEditor plugin and/or get in touch with its development team to find the best solution.
And thanks a lot for reporting an interesting issue!

update from May 31, 2018
I have tested the extension with CKEdit 4 and CKEdit 5 in standard configurations. Works perfectly with both.
The problem must be with that specific CKEdit plugin at phplist.