enther this in the terminal and restart.

csrutil disable

This does disable security, so use the csrutil enable command when you are done, it can test your theory however.

My iMac pro has now gotten into a state where I cannot boot Catalina from an external USB drive (created by the createinstallmedia tool inside the install Catalina app). The same USB drive boots from my MacBook pro. Secure boot is turned off or on and allow external boot is turned on. I suspect the latest security patch updated bridgeos or something else in the T2 chip that prevents booting anything but big sur. 

So you need to update the driver.

Also, after system re-install

Posted by: @tksonix-uk

Re-install the system (which in my case revert that update and remove the softraid driver)

which is incorrect, opening up softraid shows driver version v5.6.9 installed, so might be incompatibility between the latest security path and driver v6?

Have not seen this yet, but I can pay attention and perhaps try to reproduce this. If Big Sur fixes this however, it won't be fixed in any Catalina updates.

I've been troubleshooting this problem for last two days, and here is my findings:

• This problem seems to appear after applying the latest security update, i.e. Security Update 2021-002 Catalina. 
• Re-install the system (which in my case revert that update and remove the softraid driver) solves the problem.
• Booting into recovery and finding local users to decrypt the disk takes ages to load.
• If you wait long enough in the login screen (maybe more than 1 minutes) before entering the password, it has a chance booting into the system, but not granted.
• On failed boot, it automatically re-boots me into my bootcamp, but bootcamp does not boot as well.
• Resetting SMC temporarily solves the problem but appears again the next day.
• Turning off file vault solves the problem (even though my machine has the T2 chip).

My machine is Mac Pro 2019 just for reference, and I install my macOS and bootcamp in internal ssd. Secure boot and external boot are all disabled.

I don't think it's the softraid driver that's to blame. I discovered that reinstalling Catalina (without erasing the boot drive) allows the machine to boot up after the install. However, the same problem reappears after the next reboot. 

After making a time machine backup I erased the drive and reinstalled Catalina and I thought that resolved the problem. Then when I enabled FileVault the machine refused to boot again. I contacted apple support and they asked me to upgrade to big sure and that fixed the problem. However, I have software that I need that don't work with big sur. So unless apple can fix this it looks like I can't use filevault. 

My guess was incorrect.

Were you running SoftRAID before this?

If not, did you disable Secure Boot?