After updating to 6.1.20 on my macOS 10.15.7 host, my Solaris 10 and 11 VMs consistently aborted on startup.
Alas, I don't have further details as I promptly reverted to 6.1.18 and its extension pack; that worked fine again, as it had before. (my most immediate concern was that I have daily activities (updates, mostly) that use the VMs along with other hosts and VMs, and didn't want to leave a problem lying around for later, when I might be operating on mental autopilot)
VMs abort on 6.1.20 under Catalina (10.15.7)
Re: VMs abort on 6.1.20 under Catalina (10.15.7)
I'm getting something similar under Mac OS 10.13 (High Sierra). A working (under 6.1.18) Windows 2000 VM aborts as soon as I try to install the 6.1.20 Guest Additions. I didn't try any others - just downgraded to 6.1.18 again and it started working fine again.