As for “what does it mean?” that part is in the output you shared:
[CouchbaseLite] [Thread Pool Worker]| [Replicator] (WebSocketWrapper) [Thread Pool Worker (15)] Error validating TLS chain: RemoteCertificateChainErrors
The first thing that comes to mind is that your Android device doesn’t have enough information to reconstruct the chain of trust (i.e. it is receiving a certificate that doesn’t map back in some way to one of its trusted CA). I also notice that you are using Let’s Encrypt and if you don’t already know, one of their roots expired at the end of September 2021, so that’s also something to double check on your device.