When upgrading to 5.5.1 on WP 4.98, we discovered that three of our customized reports were no longer working with errors along the lines of "cannot connect to database." The common thread across the three was calling on activities.
This has not happened with other updates. On inspection of the three report definitions, all were missing the DEVELOPMENT tab.
Our solution was to recreate the three report definitions from scratch (easy to do since the non-working versions were still present. The newly created versions worked correctly with the DEVELOPMENT tab again in place as well.
Posted as FYI re: tip if reports are corrupt after upgrading or re: possible bug.