If you revert to 1.51 on the command base, the LCS SER2 and any other LCS connected device will stop working.
Thanks for the photo of the Manco booster. If you could post a hand-drawn block diagram of all the pieces, including the command base, all serial devices and all serial cabling it would help me give better advice.
That's really interesting as everything worked perfectly with the base having 1.51 in it and the remote 1.52. It wasn't until I did the upgrade that the problem surfaced. I ran the trains for about an hour both Legacy and TMCC and all commands worked from the computer or the remote. I even set up 2 switch routes (setup with the Etrain) that also worked from the ETrain computer software.
Ron