Apple released new version of macOS 11.0 Big Sur on 12th November 2020. Current BC Vault application (v1.4.2) does not run on the Big Sur as there are still incompatibilities with major libraries as QT that BC Vault also uses.
We apologise for the inconvenience and are investigating the issue and will hopefully be able to release new version of the application ASAP.
UPDATE 18th Nov 2020: Issue was identified and is fixed in v1.5 release that will be released in 1-2 days!
UPDATE 20th Nov 2020: v1.5.0 was just released! Download and update here.