That is a bug by the ircd which rejects a command which has been accepted for years. The outbound command comes from the popups.ini command:
.Voice:/mode # +vvv $$1 $2 $3
Since only 1 nick is highlighted, the $2 and $3 are null. The 2nd and 3rd 'v' mode should be ignored in the absence of a 2nd and 3rd nick. To placate an ircd which doesn't accept a voice command which has been valid for decades, the line in popups.ini would need to be replaced with:
.Voice:/mode # + $+ $str(v,$0) $$1 $2 $3
This replacement changes the command to have the v replicated to match the number of nicks following it. You could even replace append $4 $5 $6 depending on how many mode commands can be stacked together at the networks you use.