"Has anyone found a way to fix this issue?"

Which part? You described a lot, but didn't point out your issue to be resolved.

1. I'm pretty sure MSXML 6 is the correct substitute for 4.0. I'm pretty sure MSXML 4.0 reached end of life in 2014. And I'm pretty sure 6.0 has the components needed to run 4.0-required programs.

2. I know that QB 2015 reached end of life in May 2018. The only currently supported QB programs are 2021, 2020, 2019 and 2018. QB 2018 reaches end of life in May 2021.

Did you confirm which MSXML are installed, do you have or need any QB installed, do you know if Lacerte 2016 won't run with 6.0 and needs only 4.0?