A larger meta-issue with using the legacy "samba schema" + LDAP is that the functionality it relies on will be completely removed from upstream samba in probably version 4.14.
This means that if you're relying on this for SMB access, then you're looking at migrating to AD or LDAP/kerberos for SMB access in 2-3 years. I'm not sure what jumpcloud's plans are with regard to this. Starting in one of the later 12.0 releases (possibly U1) we will add a deprecation warning to the GUI so that users have a couple of years heads-up on a potential need to migrate directory services.
Regarding the error message, it's pretty straightforward. ldap_result(3) is failing. Possibly it's a network issue. Gut reaction is that it's probably on their end. I believe I added nscd to the 11.3 build at some point, and so you can try manually configuring that to reduce the amount of lookups to jumpcloud. I will probably add configuration for nscd to one of the later 12.0 releases (maybe U1 or U2). Interplay between caching mechanisms (nscd and winbind) can be complex and so I prefer to avoid having to do this.