Hello! I noticed the LSP was not working when you ran the Theia-vscode side of things.
To solve this I downgraded the vscode-extension-self-contained/package.json
"vscode-languageclient": "^8.0.1"
to
"vscode-languageclient": "6.1.4"
Then had to update the extension.ts imports to:
import { LanguageClient, LanguageClientOptions, ServerOptions } from 'vscode-languageclient';
Figured I'd post this here to let you know!