Install Comodo SSL, now HTTPS delivers 400 Bad Request

[ralbrightii]

So I have a site that was operating great with Let's Encrypt, but every now and then would throw out errors complaining about that it was a self-signed certificate. So I decided to add a PositiveSSL from Comodo to that domain. For WHATEVER reason, now when I visit pretty much any site on my server with HTTPS I get the 400 Bad Request error. I have tried removing and re-adding domains in VestaCP. I did all of the client-side steps (removed cookies/cache).

But I think this is a server configuration issue, something I fowled up, now I am scratching my head while banging it into the wall. I really do not want to start over from scratch.

Any ideas?

[ralbrightii]

To help those that come after, especially for VestaCP: to fix this error it is likely caused by server misconfiguration, or you are needing to update large_client_header_buffers.

This is done quite easily in VestaCP. Go to your Control Panel, click on Server, drop down to NGINX, and click on configure.
Click on Advanced options
find in the file:
large_client_header_buffers

It will likely be listed with 4 4k; change this to 4 16k or 4 32k;

Save, restart NGINX and you are fixed.