Unable to add ports to LAG

Maybe attempt the following instead.

If switches work fine with one port connected between them without LAG settings on, then I suggest you do some more troubleshooting detailed below.

1. When the LAG is enabled,  check the LAG status.   If it is down, we have negotiates issues since both sides need to be configured for LAG.  On Juniper side also verify the LAG configuration shows up or down.

2. If LAG is up but traffic won't go then check the STP state to make sure neigther side is BLK (if you using MST or PVST) then make sure to check STP per vlan that is tagged on the port to verify they are are not in BLK state even though LAG is up.

Here is steps I normally do to add a LAG memeber.

1. Reset the port that will be LAG member to default state.  Meaning PVID 1 and participation vlan 1 only.   Change the mode of port to General instead of access or trunking mode.  

2. Add the port as member of the LAG.  

3. Configure the LAG setting to match on both sides (Static vs Dynamic (LACP), matching hash mode)

4. Configure the LAG port with required settings (PVID, participation, tagging, trunking, etc)

5. Connection or no shutdown the port to bring it up.. and see if LAG is up.  

6. If LAG is down, I limit the membership to single port only and make sure LAG comes up and traffic passes.   If LAG is up but traffic fails to pass, I verify membership of LAG .. show vlan x  shows particaption and tagging status.  If LAG is down but port is up, I look at port status (after resetting the counters) to see if there is traffic going across.  

7.  For older switches, I save config and reboot them one a time on each side to see if that fixes them.  

8.  I also make sure atleast one side is active member for dynamic LAG (LACP)

Beyond this, I suggest opening a support case as this will need to troubleshoot by debugging further.. 

debug commands are on the switches to dig beyond this level.