No, any code in your Uplink function is running in your Uplink program, not on the Anvil Server.
But you have found a way to narrow down the location of your coding error. Think about it: any code after the return doesn’t get executed, so it doesn’t get a chance to encounter the problem.
It’s time to start checking the type of value that each variable and subexpression has, both in the case where the code fails, and where it works.
You’re going to see differences between the fails cases and the works cases. Then you can try to figure out why those differences are occurring. It may be that the module containing your anvil.server.callable function is being initialized differently in those cases.