You know, in the Quran Allah says "O you who have believed, do not follow the
footsteps of Satan." (24:21)
You might be sitting and talking with the other person about professional stuff but maybe something else is inside your heart deep down.
I never know what should be the ruling on this matter as the situation is quite complex. People who say that there is no problem with male and female mingling, also come up with strong arguments.
However I do know that when prophet Muhammad (SAW) used to answer the questions of the Muslims of Medina, Men and women used to come separately, or there used to be a partition between the two sexes.
During the time of the Caliph Umar (R.A), the official responsible to maintain the prices of commodities in the markets of Medina was a woman named Al-Shifa. So this means that she obviously had to deal with the clever people of the market on a daily basis professionally. So hence we cannot deny women their rights in the workforce of the country however casual mingling is a rather complex and tricky problem.
here is this beautiful article about Hazrat Al-Shifa you might want to give a quick read:
https://www.arabnews.com/node/396260
Rest, only some knowledgeable religious scholar might know what is the best decision in this regard. Islam is a religion for the rest of time till Qiyamat. If we need an answer for this simple problem, it must be somewhere in the Quran or Sunnah. Hope we find it soon.