AdiIRC

Thank you for replying.

I followed the steps and unfortunately the issue remains.
However, I just did the following:

• as AdiIRC is running, rightclick on the blank statusbar icon, then rightclick again on the top displayed 'AdiIRC' entry (it's actually this shortcut file: C:\Users\Kostas\AppData\Roaming\Microsoft\Windows\Start Menu\Programs\AdiIRC.lnk) and click "Properties"
• now in 'Shortcut' tab, click on the "Change icon" button:
  it shows a "Change Icon" window saying: Windows can't find the file %USERPROFILE%"\AppData\Local\Temp\AdiIRCIcon.ico :
  

So, I clicked the "Change icon" button again, and changed its path to "C:\Program Files\AdiIRC\AdiIRC.exe", selected the listed icon and the issue is now fixed.

Maybe this situation occured during a AdiIRC beta update??
I'm saying this because, a few times Windows Defender has wrongly found a beta update as "threat", (I've noticed that AdiIRC uses USERPROFILE"\AppData\Local\Temp to download a file which then replaces AdiIRC.exe)
and then I have to whitelist+restore it in Windows Defender, for the update to complete.
Maybe in such cases the file that AdiIRC downloads in Temp, becomes eventually removed/missing from the Temp folder, and so AdiIRC doesn't have an icon in taskbar on Windows launch?

it's possible that windows refused to load the icon from the exe file when it was flagged and instead used a plain icon in it's place which is now cached somewhere, clearing the windows icon cache should fix such issues, I'm not exactly sure how that is done.

You'll have to ask someone who knows more about windows than me, maybe ##windows on freenode.