Same here. What I did:
Added button below Text box. Saved. Re-opened. No problems.
Changed button height to 15px, color to #660000 and description. Saved it. Re-opened and got the '-->' and '!--" around the button. See scrn print. Multiple saves and re-opens will give nested characters.
Pleas revise Button save code, it seems that the script tags are not removed / replaced correctly.
By the way, this is an installation of Acymailing 6 next to existing version 5 and I used the migration to migrate all data from the 5 to the 6 installation.