In newer of versions of Virtualbox, especially above 6.0 (eg. 6.1 like the example below), a lot of times the "Enable Nested VT-x/AMD-V". If you are having this issue, you will see the option is grayed out. It doesn't mean that your computer does not support virtualization, although it is possible it is disabled in the BIOS. You can verify........

In Thunderbird in a very large folder suddenly all of my e-mail list became blank/white even though you could click on the invisible/white/blank lines and it would show the e-mails. Itried to close and reopen but only going to Folder -> Properties and clicking Repair Folder fixed it (basically it had to redownload/rebuild the entire index and all e-mails).........