Deleting bin and obj folders, cleaning solution, and restarting studio did not help. Designer does open a base form. Inherited form (FrmBase) has an empty constructor and no parameters. Some of the derived forms have and some do not have an empty constructor, constructor with or without parameters, the error is the same for all. FrmBase is in another project.

Update 2022-08-05: when I copy the exact form file with its designer, change the name of the class and it works.