save screen buffer to file

screen -r myscreen

Go to copy mode
Ctrl + a [

Go to the beginning of the line that you want to copy (use arrow keys or Pg Up)

then press Spacebar to initiate copying of current line

Go to the end of the line that you want to copy

press Spacebar again to stop copying of the current line

go to command mode
Ctrl + a :

then Save the selected text to the file by:
writebuf /home/bla/screen.log