When trying to open the VTB, it leads to the error message "an unknown error has occurred".
Investigate this issue by checking the card available on the VTB.
This can be done by opening the VTB Board record in "vtb_board" table.
In the "Visual Task Board Cards" related list of the record, all the cards of the VTB would be visible.
Set the cards as "removed=true" and test if that fixes the issue.
Ultimately there maybe 1 or more than 1 cards which when marked as "removed=true" would fix the issue.
Once the problematic card is removed from the VTB by setting "removed=true" it would fix the issue.