Recently upgraded to 2021.03 and have been upgrading components from the SPM. After upgrading Servoy Extra Components, Bootstrap Extra Components, Key Listener and Servoy NG Grids all forms (css position layout) displayed in developer as a blank. Started to downgrade the components and got lucky as the first one i tried fixed the problem, took Servoy Extra Components back to 4.0.0 as forms now display correctly in developer.
Servoy Version: 2021.3.0.3642
Windows 10 Pro, 21H1, OS Build 19043.1023
Richard