Thanks for your answer.
But, since I never did anything (like this) through TWRP, can you tell me how to 'mount it as writiable' ? (mount... what? 'system' ?) Sorry for stupid questions but...I just never did such thing before and I have no clue.
In the past I used File Explorer (once I rooted the phone), or nowdays MiXplorer to do things. But thats from within Android OS. Never did change things (moved, pasted, deleted system files) through TWRP.