The Zoiper window can appear "transparent" whenever the color depth of the display is set to 16-bit or when you are using RDP with quality options that allow dynamic changes to the color depth with bandwidth preservation purposes.

To work around this issue you need to set your display to "True color (32-bit)".

To do so, please go to your Desktop -> Right click on it -> select "Screen resolution" -> select "Advanced settings" -> go to the "Monitor" tab -> search for "Colors: " with a drop-down list, and change the color depth to 32 bit -> click "OK".

If this issue appears on RDP session, you need to disable the automatic image quality options depending on the network bandwidth from the RDP client.