Arturia Piano v3 blank FX window

[rupert]

When I load Arturia Piano v3 vst3 it comes up with a blank FX window not showing the VST GUI. I can still play it and hear it but without the UI I can't adjust any parameters. If I try to close the Window or remove it from the FX then Reaper becomes unresponsive and I have to kill the process. Clicking the UI button also causes Reaper to freeze. It also does this with Arturia Analog Lab V and Piano v2. All of these work correctly if I use FL Studio.
It seems to work if I start with a blank project then add it. I can see the VST UI but if I save the project then try to reopen it the FX Window is blank when I open it and Reaper will stop responding if I click on any of the FX window elements.

Reaper 6.57
Windows 10 Version 10.0.19044 Build 19044
32 GB memory