You might have read our warning from 29th April, that the plain AutoCAD Update 2021.1.1 has caused crashes in AutoCAD Plant 3D 2021. Good news: This issue is solved. In the new Plant 3D Update 2021.1, which will be released today. Also, the new Plant 3D 2021.1 patch itself has integrated the ACAD 2021.1.1 patch. So no need for you to install twice.