Jupyter can't connect to kernel.

I am running jupyter on a digital ocean ubuntu 18.04 VM. I'm using an Apache reverse proxy to map port 8888 to `jupyter.myserver.com`.

I know my Apache proxy is working because:
1. I have been running jupyter notebooks successfully until the last few days.
2. I have other apps (such as rstudio server) I am accessing on different ports with identical reverse proxies.

I know it is not any kind of tcp error on my local machine because:
1. I have tried turning off all firewalls
2. I have tried turning off all antivirus programs
3. I have tried four different browsers, including in safe mode and anonymous windows.
4. I have tried using FF in my WSL linux installation
5. I can run a notebook successfully on localhost
6. I have tried other computers and even my phone.

Symptoms:
1. I connect to the notebook (either as `/tree?` or `/lab`.
1. I try opening a new terminal. The terminal tab opens, but no prompt appears.
1. I try opening a notebook and executing something like `print("hello")`. The cell never executes.
1. I see the broken link symbol to the right of the kernel's name and the orange "Connecting to kernel" message.
1. No amount of disconnecting, reconnecting, kernel restarting, opening, closing, trying different kernels, or any other games changes the result.
1. I get 404 errors in the console where I started the notebook, but no other errors.

```
[W 04:56:54.446 NotebookApp] 400 GET /api/kernels/f56f696b-4ccc-4038-9674-
993d6cbe96be/channels?session_id=e6f4e169ac134aed814f87efc12a23b3 (127.0.0.1) 4.13ms
referer=None
[W 04:56:55.769 NotebookApp] Replacing stale connection: f56f696b-4ccc-4038-9674-
993d6cbe96be:e6f4e169ac134aed814f87efc12a23b3
[W 04:57:02.270 NotebookApp] Replacing stale connection: f56f696b-4ccc-4038-9674-
993d6cbe96be:6766844e2fc0474080895aeb758a5e6b
[W 04:57:55.742 NotebookApp] Replacing stale connection: f56f696b-4ccc-4038-9674-
993d6cbe96be:6766844e2fc0474080895aeb758a5e6b
```
1. There are errors in `/var/log/apache2/error.log`:

```
[Thu Sep 13 04:59:12.471533 2018] [proxy_http:error] [pid 8510] (70007)The timeout specified has expired: [client 73.11.33.164:38901] AH01102: error reading status line from remote server localhost:
8888
[Thu Sep 13 04:59:12.471597 2018] [proxy:error] [pid 8510] [client 73.11.33.164:38901] AH00898: Error reading from remote server returned by /api/kernels/f56f696b-4ccc-4038-9674-993d6cbe96be/channel
s
```