I am having this very problem, but have confirmed the icons I am failing to upload are PNG files and all settings mentioned above are in effect

Our TS3 is set up as follows

There are multiple Officer Server Groups. Each Officer is given an Office Server Group as well

Captain in Recruitment or Lieutenant in Combat for example

Each...

I have several Channels in TS3:

Recruitment Office
Intelligence Office
Combat Office
etc.
I have Officers tagged with certain Server Groups:

Recruitment Officer
Intel Officer