I haven't seen a resolution so forgive me if you've already fixed it.
Does the problem go away if you change from "security = user" to "security=share"?
It's been several years since I dealt with Win9x but if I recall correctly 9x only supported share security in a workgroup. You had to be logged in to a domain (nt authentication) to use user security. To do that would mean setting up one of the ubuntu machines as a PDC and that's beyond the scope of this thread.
IPC$ (inter process communication) errors are usually authentication or capability errors (wrong password, wrong encryption or wrong security type.)
I seem to remember having to set "encrypt passwords = true" in the [global] section of smb.conf but thay may have changed.
If neither of the above works, then make try again using an account with a password on the win98 machine. Null passwords required a working guest account. (I'm unsure if one is set up by the ubuntu installation, nobody used to be the default)
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