My memorized reports that were customized - standard reports modified/changed and memorized - did not open in 2021.  I had to rebuild those reports.  For example:  "Trial Balance - Cash accounts only" - will not open.

My memorized reports that were standard reports, just memorized, worked.  For example "Trial Balance" report memorized, did open with 2021.

I found it to be the modified reports that were memorized that did not work in 2021.  I am having to rebuild those reports and re-memorize with a new name, then delete the original report that would not open with 2021.

And by the way - I tried all the above steps that were recommended including the update link and rebuild list.  None of those steps worked.