'The function runtime is unable to start.' after building template in Sandbox

#### Check for a solution in the Azure portal
The `Diagnose and solve problems` blade in the azure portal just loads forever (20min, doesnt make sense to wait even longer).

#### Investigative information

Please provide the following:

- Timestamp: 2019-07-22T08:51:15.369Z
- Function App version (1.0 or 2.0): 2.0
- Function App name: timer-test-mr1990
- Function name(s) (as appropriate): TimerTrigger1
- Invocation ID:
- Region: West Europe
- Session Id: bd08575526ce4ba0b6c326f7608a9a50

#### Repro steps

Provide the steps required to reproduce the problem:

1. Go to Microsoft Learn: https://docs.microsoft.com/en-us/learn/modules/execute-azure-function-with-triggers/4-create-timer-trigger
2. Activate Sandbox
3. Create fucntion app with timer triggered function in sandbox, In-Portal
4. As soon as the new function shows, the error message appears to the top right

#### Expected behavior

Function gets created, can be modified, runs according to Cron-setting.

#### Actual behavior

Error message appears to the top right and nothing happens, updates to the function are saved but aren't executed either:

The function runtime is unable to start.

Session Id: bd08575526ce4ba0b6c326f7608a9a50

Timestamp: 2019-07-22T08:51:15.369Z

#### Known workarounds

None, recreating doesnt work. The MS-Learn Module can be finished, but the function doesn't work.

#### Related information

-

Start with the Microsoft Learn timer-trigger sandbox steps and reproduce the failure using the reported timestamp, region, session ID, and Function App details. Investigate the Azure Functions host startup diagnostics; done means the sandbox-created timer function starts, accepts updates, and executes according to its cron setting.

Written by the indexing model from the issue text.