EF. The connection was not closed. The connection's current state is connecting

Trying to use a connection that is already "connecting" - clear sign of some race condition.

1. Re-check that IUserService is registered with "scope" lifetime, and all it dependencies (userManager, dbContext) too
2. Do not use IServiceProvider you obtained during app startup for scope-bases services resolution - it is NOT related to current request scope and return instances from "some other universe". Use HttpContext.RequestServices for service resolution.
3. Check that your are "awaiting" all async methods. If you start second request while still executing first one - you may possibly "catch" dbContext during "connecting" stage.
4. Your JwtMessageHandler instance is one/single per app. So don't use it's property for storing _userService (remove private IUserService _userService). Instead, use local variable inside OnMessageReceived (var _userService = ...).

You already checked (1), (2) and (3). I think (4) is the last one you need to fix your bug.