Hi Mark,
I haven't used Solaris, but it's possible that it's default configuration (or
some additional configuration) prevents the use of RSA. Based on the error, "RSA key
is not allowed", it seems likely that RSA-keys are not allowed at all or there is a
minimum key length (3072, 4096) and you are testing with at 2048-bit key.
Have you tried generating a different type of ssh key, e.g. ed25519 and testing that? or
checked your sshd_config for the PubkeyAcceptedKeyTypes option?
Not sure if that's helpful, but I wish you luck.
Best,
Owen