Seems like when an interface, in this case L2TP Client interface, has been disabled for a while or any time really, winbox gives me an error say the interface doesn't exist after attempting to enable it, I will close the error window and try again and it enables without any issues, seems to intermittent but happened atleast 8~10times.

on 6.42.2

Yeap, I've seen this too.

But I think it is solved by 6.42.5. I haven't seen it for a while now.