I had the same problem upgrading from 2018 to 2021.  Tried all of the suggestions, nothing worked.  Support told me I needed to go from 2018 to 2019 and then 2019 to 2020 and then 2020 to 2021.  I spent hours going through that process & the reports didn't work in 2019 either.  It may be a 2018 issue.  I am going to try to resort list, verify & rebuild in 2018 version & try again.  Fortunately I found out quickly before I had the chance to go back to 2018.  Many many hours wasted!!! Very frustrating!