TYPO3 Core

These steps should be sufficient to reproduce on the latest master:
1) Enable and load FSC
2) TS Setup:

page=PAGE
page.10 = FLUIDTEMPLATE
page.10 {
  templateName = Default
  layoutRootPaths {
          10 = fileadmin/Layouts
   }
  partialRootPaths {
          10 = fileadmin/Partials
  }
  templateRootPaths {
          10 = fileadmin/Templates
  }
}

3) put a Default.html inside /fileadmin/Templates/ with the following content:

<f:layout name="Default" />
<f:section name="content">
<h1>hello world</h1>
</f:section>

4) put a file Default.html inside /fileadmin/Layouts/ with the following content

<f:render section="content" />

5) Result: the page.10 object is ignored and instead you get inside the page:

<div id="c1" class="frame frame- frame-type- frame-layout-1"></div>

If you change the layout name (e.g. "Index.html") and the related <f:layout name="Index" /> everything works again.

UPDATE

1) to reproduce the issue, you must have a configuration like this one:

page.10 = FLUIDTEMPLATE
page.10 {
  templateName = Default
  layoutRootPaths {
          10 = fileadmin/Layouts
   }
  partialRootPaths {
          10 = fileadmin/Partials
  }
  templateRootPaths {
          10 = fileadmin/Templates
  }
  variables {
   maincontent < styles.content.get
  }
}

2) If you use a subfolder as repository for the Layouts, e.g.

  partialRootPaths {
          10 = fileadmin/Layouts/Page

the issue is still present

3) The Default.html of FSC is taken instead of that inside fileadmin/Layouts/Page: if you modify the template:

<f:layout name="Default" />
<f:section name="Main">
<h1>hello world</h1>
</f:section>

you are able to see the content, but you are using the Default.html layout file of FSC: you can even still keep the layout with the wrong section name:

<f:render section="content" />

4) the responsible of the issue is the variable maincontent < styles.content.get
If you remove that line, and use instead:

 lib.content = < styles.content.get

in combination with <f:cObject typoscriptObjectPath="lib.content" />, everything works fine

I'm using this configuration:

lib.templates.base.file.stdWrap.cObject = CASE
lib.templates.base.file.stdWrap.cObject {
    key.field = backend_layout
    key.ifEmpty.data = levelfield : -1 , backend_layout_next_level, slide

    default = TEXT
    default.value = EXT:project/Templates/BackendLayouts/Default.html

    pagets__Home = TEXT
    pagets__Home.value = EXT:project/Templates/BackendLayouts/Home.html
}

And currently (8.6) I get no output in styles.content.get when fluid_styled_content is included to template. CSS Styled Content works.
Most likely related to this topic?

@Armin: Your are running into https://forge.typo3.org/issues/80044

I got also no output.

After applying this Patch https://forge.typo3.org/issues/80044

and renaming layout name "Default" it works.

Are you sure this is against latest master?

I don't get it that the FLUIDTEMPLATE declared in page.10 is inheriting stuff from FSC.

I've just tried to reproduce this. New installation on master, with only the TS as mentioned and the template and layout file. FSC installed.

Cannot reproduce this. Body content is as expected.

<h1>hello world</h1>

Looking at your first TS setup, there is an error

page=PAGE
page.10 = FLUIDTEMPLATE
lib.stdcontent {
  templateName = Default
  layoutRootPaths {
          10 = fileadmin/Layouts
   }
  partialRootPaths {
          10 = fileadmin/Partials
  }
  templateRootPaths {
          10 = fileadmin/Templates
  }
}

lib.stdcontent should be page.10. But with that error I can't explain why something is rendering at all

Exact the same problem still in 8.7.1 with this code:

page.10 = FLUIDTEMPLATE
page.10 {
    layoutRootPath = EXT:myext/Resources/Private/Layouts/
    partialRootPath = EXT:myext/Resources/Private/Partials/

    file = EXT:myext/Resources/Private/Templates/Default.html

I had a layout file in Resources/Private/Layouts/Default.html

Renaming it to Standard.html (or something else) solved it.

Same here, took me hours!

I think this is a common problem upgrading from 7 LTS, because examples on the web using <f:layout name="Default" />.

yes, same here.

Renaming the file solved the Problem.

I can confirm this problem.
I think this is a caching issue.
Just one Fluid layout from StandaloneView per filename is cached as a php file to typo3temp/var/Cache/Code/fluid_template/layout_Default_xxx.php
This cached layout is used for every FLUIDTEMPLATE / StandaloneView.

This Issue is also affecting partials in my case. Naming a partial for example ../Partials/Header/Header.html (same as fsc names it) results in no output of the selfmade partial.

I can confirm the issue and the "fix" of renaming own Layout files to something other than Default.html on TYPO3 8.7.1 & 8.7.2

Same here TYPO3 8.7.4
Renaming Layout "Default" to "Main" solved it.

PROBLEM is even the ExtensionBuilder creates Layout called "Default"...and I think I followed this "convention" in all projects...and all of us do ;-)

Since it will appear also with other names like naming a partial "Header" will cause the same problem - this should be fixed!!

Dear TYPO3 Core team! Can you give us a bit of an insight if somebody is working on this issue?

It seems, there are a couple of related bug-reports, some with the 'magic words' Template-name Default.html. I also found people having troubles on german-speaking forums.
e.g. https://webdesign-forum.net/thread-4176-post-18883.html
and https://www.typo3.net/forum/thematik/zeige/kategorie/linux-unix/thema/126195/

Patch works in my situation (https://forge.typo3.org/issues/82889), which should be the same problem as described here.

Patch set 2 for branch TYPO3_8-7 of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/55252

TYPO3 8.7.9 was released in December, whereas this bug was fixed a week ago, so if you want to test whether or not the issue have been fixed, you need to fetch the TYPO3 8-7 branch from git or wait until the next patch release - which according to the roadmap should be released on the 6th of february.
https://typo3.org/news/article/typo3-release-improvements-v8-maintenance-release-schedule/

OK, the patch worked for me.
And renaming the partial header.html to Header.html worked for me too.