VNC connections not working to MacOS 15.1 (Sequoia)

Jump Desktop v8.10.8 (MacOS 14.6.1),  Jump Connector v6.10.37 (MacOS 14.3 and 15.1)

1) High-level problem:  I have a local network containing two Mac Minis (Mac 1: OS 14.3 and Mac 2: OS 15.1), behind a router with a single public dynamic IP address.  I want to be able to Jump Desktop to either one.  I understand how to set up port forwarding to get <IP>:5900 to go to 5900 on one internal mac and <IP>:5901 to go to 5900 on another internal mac.  But the second port forward isn't working - at least Jump Desktop won't open a window into Mac 2 but it does open a window into Mac 1.  So in the next step, I tried testing locally, not coming from the outside.  

More specifically, the failure for Mac 2 is ..... A small window open with a login panel, but even though I enter valid credentials, the login window goes away for a second, and then returns, as if the credentials have failed.  This loops forever until I click Cancel.

2) From my local MacBook, Jump Desktop automatically creates successful Fluid connections to both local MacMinis, and when I turn on VNC, also automatically creates VNC connections.  But, although I can access Mac 1 through both Fluid and VNC connections from the MacBook, I can only access the Fluid connection for Mac 2.   When I access the automatic VNC connection for Mac 2, I get the same results as the failed Mac 2 in Issue #1.... small login window which ignores valid credentials.

3) I tried creating VNC connections manually but had the same result as #2.  Mac 1 works, Mac 2 does not.  I've spent over 8 hours verifying the two Macs are identically set up, except for the OS version being different.

4) I would be happy to create a Fluid link, instead of VNC, but there seems to be no way to do that.  From Jump Desktop, when I go to add a new link, the only choices I am given are RDP and VNC, not Fluid.  So I have no way to test if I can create a Fluid link to two different ports, as I would have to do if I were to come in from outside the router.

My best guess is that there is some new security measure in MacOS 15.1 (different from 14.x) that is not allowing me to use Jump Desktop to connect, but for some reason,  the automatic Fluid connection works, while automatic or manual VNC connections do not.  Do  you know of anything like "You must have a password" that is preventing VNC connections?  

One other symptom.... after shutting down Jump Desktop and reopening it (or even just going away for a while) Mac 1 never starts a screen saver  or lock the system, so it comes right back to the active session.  But Mac 2 (the one that isn't working with VNC) always comes back to a screen-saver-password-page.  Both are set up to time out after 5 minutes, as far as I can tell.

Hoped for resolutions: 
A) Just tell me what to do to connect to 2 macs behind a single router, OR 
B) Help me find out why VNC connections are not working with MacOS 15.1 and I can do the router stuff.

Please help...  I'm totally stuck here.