I also believe this is probably a Proton-specific issue.
I am on Arch linux
If i launch EVE via Steam (hence Proton) i experience the mentioned “Skill Plan service is not available” issue, as well as some missing Icons regarding sov nullsec, probably another gRPC feature @CCP_Caffeine talked about?
If i launch EVE via the evelauncher or evesetup tools from the AUR, then using wine staging 7.2 i experience no issues.