I’ve discovered what might be happening, but not sure how to fix it. I use a VPN for all browsing (Mullvad) but have added the EVE executable and the Launcher to the split tunneling excluded apps so they don’t run through the VPN. I disabled the VPN completely just now, logged in to the game and tadaaa, no error message and the personal skill plan function was available.
I re-enabled the VPN and went through every single application in the EVE folders and added them one by one to the split tunneling exceptions list but the problem remained. So there must be some element within the main EVE executable that is accessing the internet separately for the skill plan service.