I am having the same issue online with the totalling of boxes 68520 to 68560 of the T1198. However, if I remove my amount from box 68555 (5th prior year - 2019) and recalculate the totals the form works and allows me to move on. The only difference with box 68555 for me is that the amount entered is under $1000 while all others are over $1000. I also tried adding the amount from box 68555 to another year so that the totals match my paperwork and the error reappears, so it might be something to do with the total amount of all the boxes. Greater than $xxxxx.xx no go but less than $xxxxx.xx then the T1198 works...
