It was Node 15. Downgrading to Node 14 worked. On a Mac, that's
rm -rf /usr/local/bin/node
rm -rf /usr/local/bin/npm

and then reinstalling Node 14.
I created two test servers, one with typescript and one with javascript.
For typescript, I had to run npm install first, then npm start, whereas with javascript npm start worked immediately.

Cheers!

Hello,
the Quickstart documentation says:
"Create a bare-bones Colyseus server by using npm init colyseus-app.
npm init colyseus-app my-colyseus-server
cd my-colyseus-server
npm start"

I did that and got this error:

my-app@1.0.0 start
node src/index.js

node:internal/modules/cjs/loader:928
throw err;
^

Error: Cannot find module 'express'
Require stack:

• /Users/jack/test/colyseus/my-colyseus-app/src/index.js
  at Function.Module._resolveFilename (node:internal/modules/cjs/loader:925:15)
  at Function.Module._load (node:internal/modules/cjs/loader:769:27)
  at Module.require (node:internal/modules/cjs/loader:997:19)
  at require (node:internal/modules/cjs/helpers:92:18)
  at Object.<anonymous> (/Users/jack/test/colyseus/my-colyseus-app/src/index.js:2:17)
  at Module._compile (node:internal/modules/cjs/loader:1108:14)
  at Object.Module._extensions..js (node:internal/modules/cjs/loader:1137:10)
  at Module.load (node:internal/modules/cjs/loader:973:32)
  at Function.Module._load (node:internal/modules/cjs/loader:813:14)
  at Function.executeUserEntryPoint [as runMain] (node:internal/modules/run_main:76:12) {
  code: 'MODULE_NOT_FOUND',
  requireStack: [
  '/Users/jack/test/colyseus/my-colyseus-app/src/index.js'
  ]
  }
  npm ERR! code 1
  npm ERR! path /Users/jack/test/colyseus/my-colyseus-app
  npm ERR! command failed
  npm ERR! command sh -c node src/index.js

I then tried to install express with npm install and the package.json:
npm ERR! code ERESOLVE
npm ERR! ERESOLVE unable to resolve dependency tree
npm ERR!
npm ERR! While resolving: my-app@1.0.0
npm ERR! Found: colyseus@0.14.6
npm ERR! node_modules/colyseus
npm ERR! colyseus@"^0.14.0" from the root project
npm ERR!
npm ERR! Could not resolve dependency:
npm ERR! peer colyseus@"^0.11.0" from @colyseus/monitor@0.12.2
npm ERR! node_modules/@colyseus/monitor
npm ERR! @colyseus/monitor@"^0.12.2" from the root project
npm ERR!
npm ERR! Fix the upstream dependency conflict, or retry
npm ERR! this command with --force, or --legacy-peer-deps
npm ERR! to accept an incorrect (and potentially broken) dependency resolution.
npm ERR!
npm ERR! See /Users/jack/.npm/eresolve-report.txt for a full report.

npm ERR! A complete log of this run can be found in:
npm ERR! /Users/jack/.npm/_logs/2021-01-06T22_14_57_858Z-debug.log

I'm on MacOS BigSur.
node -v: 15.4.0
npm -v: 7.0.15
git --version: git version 2.24.3 (Apple Git-128)

I couldn't find anything in github issues and this forum, so I guess I made a beginner error and missed a simple step?

Thanks for any help!