It looks like `curY (tty) >= FrameRows (tty) - 1`:

(lldb)

frame #9: 0x0000000100173380 Emacs`cmcheckmagic(tty=0x000000010180d000) at cm.c:118

115 if (curX (tty) == FrameCols (tty))

116 {

117 if (!MagicWrap (tty) || curY (tty) >= FrameRows (tty) - 1)

-> 118 emacs_abort ();

119 if (tty->termscript)

120 putc_unlocked ('\r', tty->termscript);

121 putc_unlocked ('\r', tty->output);

(lldb) p (tty)->Wcm->cm_magicwrap

(bool_bf) $0 = true

(lldb) p (tty)->Wcm->cm_curY

(int) $1 = 2

(lldb) p (tty)->Wcm->cm_rows

(int) $2 = 3

> was the original cause also in "emacs -nw", or was it in a GUI
frame?

It was not in a GUI, but it was an `emacsclient -t` invocation.