On mIRC when you voice devoice and op deop yourself or another user when using the mIRC control function it says:

#channel v * You must specify a parameter for the voice mode. Syntax: <nick>.
#channel o * You must specify a parameter for the op mode. Syntax: <nick>.

This happens on InspIRCd 3 but does not happen on InspIRCd 2

mIRC when you are a channel op
right click on yourself or another person in the list of users in the channel.
Control -> voice / op - to voice and op the user.

And same for devoice / deop

It does give you the voice and op modes and removes them.

When you voice and op using /mode #channel + nick -- it does not get this message.

On InspIRCd they say -- This is a bug in mIRC. It's sending a bunch of extraneous prefix mode changes without any values which is triggering the syntax hint.