I had the same problem today in my Delphi 10.4.
After some searching and comparing with a newly created registry key for Delphi, I found out that it is due to the disabled entries in the IDEInsight key.
Example:
[HKEY_CURRENT_USER\SOFTWARE\Embarcadero\BDS\21.0\IDEInsight]
"Disabled0"="Project Options"
"Disabled1"="Components"
"Disabled2"="Object Inspector"
"Disabled3"="New Items"
"Disabled4"="Open Files"
"Disabled5"="Recent Files"
"Disabled6"="Recent Projects"
"Disabled7"="Component Palette"
"Disabled8"="Code Templates"
"Disabled9"="Desktop SpeedSettings"
"Disabled10"="Projects"
"Disabled11"="Files"
"Disabled12"="Forms"
"Disabled13"="Commands"
"Disabled14"="Preferences"
After deleting the complete key in the registry and restarting Delphi, IDE Insight worked again.