I just encountered the same issue in VS 2022 with a UserControl.
When I opened the corresponding .resx file of the UserControl, I found an entry with the message "Value cannot be displayed." I removed this entry from the .resx file and then was able to open the UserControl in the VS Designer (no restart needed)
The erroneous entry in the .resx file was the only one referencing mscorlib, Version=18.104.22.168. All other entries are referencing System.Windows.Forms, Version=22.214.171.124.
In the Visual Studio Output Log, there was a corresponding note: The conversion exception was: Unable to find assembly 'mscorlib, Version=126.96.36.199.
Project background Information:
The C# project was originally developed with .NET Framework 4.8, Windows Forms, and DevExpress, and was migrated to .net6-windows a year ago. I performed the migration to .net6 manually and did not use any specific tools for it. The VS version is Microsoft Visual Studio Professional 2022 (64-bit) - Version 17.6.2. The OS is Windows 10.
Perhaps this information might be helpful.