Same problem.
No errors in the logs.
However I fixed a similar problem with the Sucuri extension (same message, undefined error),
It was an obsolete hook (mainwp-sucuriscan-sites => mainwp_sucuriscan_sites).
Maybe there is also an obsolete hook in VulnChecker, that was’nt properly flagged as deprecated ?
A lot of hooks names were modified recently, with “-” replaced by “_”