Invalid argument in ThreadPool

### Step 1: Please describe your environment

* ZeroNet version: v0.7.1 (r4469)
* Operating system: Ubuntu
* Web browser: Firefox
* Tor status: available
* Opened port: yes
* Special configuration: none

### Step 2: Describe the problem:

#### Steps to reproduce:

1. No idea, will check if it can be reproduced later.

#### Observed Results:

Lots of errors like the following one appear at startup:

```
[08:05:15] - Unhandled exception: OSError: [Errno 22] Invalid argument in threadpool.py line 137
Traceback (most recent call last):
File "/usr/local/lib/python3.7/dist-packages/gevent/threadpool.py", line 137, in __run_task
thread_result.set(func(*args, **kwargs))
OSError: [Errno 22] Invalid argument
```

#### Expected Results:

No exception.