Hi there,
Likely because the initial connection at startup is not a full bind, but rather a quick connection check. Happened to me already, and also in this thread for example, so don’t be misled by that.
Right now the error is pretty clear:
I suggest double-checking credentials, and maybe validating the binding first a simple LDAP client like ldapsearch.
Also, probably worth trying without ldap.authentication=GSSAPI for the time being, to simply the configuration and troubleshoot incrementally.