TYPO3 Core

Some questions:

• Have you checked, that the configured (external) 404 pages are accessible directly for the domains 50+ ? (Show content from page -> external url or internal ?)
• Have you thought of to use a typo3 page as 404/error page and use it directly instead of requesting it with a subrequest ?
• The errorpage is something with an folder .. is it a "typo3 page?" then directly use the t3://page thing instead of external request
• Are the domains protected by basic auth ? ==> add a working auth to the error page url: http://<user>:<pass>@www.domain.tld/404-not-found
• Are the domains http or https ? if https, in your log it stated http .. try with https setup directly. Maybe it conflicts with an redirect ??
• Can you check what is mainly different in the <50 configuration to thr 50+ configurations ?
• The last one evenually resulted in a request loop if it goes to typo3 > errorhandler -> subrequest typo3 -> errorhandler -> subrequest typo3 -.- and reached a timeout

Setting something up to "test/verify" this is a task for itself ;)

Hi Stefan,

sorry for not answering earlier.
quote your questions and answer below each:

Have you checked, that the configured (external) 404 pages are accessible directly for the domains 50+ ? (Show content from page -> external url or internal ?)

yes, the pages could be called directly as long as I never implemented an own error-handler.
This applies to all domains.

Have you thought of to use a typo3 page as 404/error page and use it directly instead of requesting it with a subrequest ?

I never made subrequests but used the site-configurations to configure the error-pages.

The errorpage is something with an folder .. is it a "typo3 page?" then directly use the t3://page thing instead of external request

The site-config is writing error pages like this, it was in use.

Are the domains protected by basic auth ? ==> add a working auth to the error page url: http://&lt;user&gt;:&lt;pass&gt;@www.domain.tld/404-not-found

All domains are and have been public available.

Are the domains http or https ? if https, in your log it stated http .. try with https setup directly. Maybe it conflicts with an redirect ??

Everything is configured with https.
In frontend there haven't been any indications that http versus https caused problems.

Can you check what is mainly different in the <50 configuration to thr 50+ configurations ?

The differences are slightly only and mainly related to content only.
All domains use the same default-setup.

The last one evenually resulted in a request loop if it goes to typo3 > errorhandler > subrequest typo3 -> errorhandler -> subrequest typo3 -. and reached a timeout

I never understood what happened in detail and even remarked that curl-requests to the error-pages have been made.
Concernig "subrequest" I'm not sure if I understand what you mean exactly, perhaps you can elaborate.

Setting something up to "test/verify" this is a task for itself

Sure, that's some work, is there in ddev some option perhaps to provide an installation similar like a composer file allows installation anywhere?