As expected, TurboTax incorrectly imported QC slips; after an indepth (and tedious) review, there were two bogus T3/Rel16 that where somehow added during the import.
I removed them (after I verified that they were indeed not valid!), and the error went away. I was then able to submit my reports.
As mentioned, I have encountered this issue last year as well. This seems to be a recurring issue which many people seem to encounter year after year, and should be addressed. I will no longer use this functionality in TurboxTax.
