Setup btcpayserver-docker with the opt-add-ltcmweb
fragment.
Install the plugin (or restart BTCPayServer if already installed).
Add MWEB View Keys to Litecoin MWEB Wallet settings in BTCPayServer admin.
They can be retrieved using Electrum MWEB (release-9 and later).
Thank you, well done on getting it working. It's really odd though that nobody else previously had a problem. Perhaps it is a peculiarity of your docker setup.