HTTPS?: Form submission is not possible

[InTerestedOWL]

Hey there,
I am currently building a website with statamic.

I am now facing some issues regarding the use of https and the form submission:

For explanation I have a lokal environment and a live environment on a server.

My problem is the following: While I can submit the form on the local environment, my live environment reports me a fatal PHP Error which I can't really understand:

Error
Call to a member function lang() on null

I am not new to the game. I looked after the reason:

And found the FormController.php line 51 which is causing the issue:
$this->withLocale($site->lang(), function () use ($fields) {

Checked the $site variable it is obvious in line 34 that there is something wrong. I debugged a bit:

$site = Site::findByUrl(URL::previous());

While URL::previous() returns the previous page, which behaves as expected.
But Site::findByUrl() returns null. Which is the problem..

Since css and co was not loaded in the control panel via https. I added URL::forceSchema('https'); to the web.php channel. Maybe this is a fault?

[jasonvarga]

Can you show your config/statamic/sites.php file?

[InTerestedOWL]

I do not use cloudflare at all. The page is available freely.

Everything works as expected except for the form submission which fails as I mentioned in the FormController.

[jasonvarga]

What service have you used to get an SSL certificate?

[InTerestedOWL]

The ssl certificate is from Let's Encrypt

[InTerestedOWL]

I don't think, that we have to search the problem at the ssl certificate. I don't understand, why Site::findByUrl('https://:example.com/site') is returning null?

What have I todo that the complete page is rendered correctly over https without 404 errors and fatal php errors caused by failed finds of my URL ....

[InTerestedOWL]

Tested it to submit the formular. over the live preview from the control panel.

And surprise: There it is working correctly! It is submitted without error?! Could not be the reason. I suppose the error in the configuration of the https.