If I call gpg-connect-agent without arguments, it tries to start the agent, but fails.

If GNUPGHOME is 89 chars or longer, the error message is "gpg-connect-agent: can't connect to the agent: Filename too long".

If GNUPGHOME is between 81 and 88 chars, the error message is "gpg-connect-agent: can't connect to the agent: IPC connect call failed".

This is most likely the same problem as in T2964, i.e. sun_path in struct sockaddr_un is too short, but the solution proposed in that bug (using /var/run/user) does not work on Windows.