I had this happen on a 1041 yesterday...I hadnt even entered any income into the return, but it had Line 15b highlighted pink as an error...I exited the file and came back to it later to make entries (I figured I could at least get it completed, even if I couldn't file until that issue was fixed) and by the time I had finished entering everything, the error was gone, I have no clue what I did or didnt do to make it happen, it was really weird.