TYPO3 Core

So what has been configured as cookiedomain and what is the server sending?

Without any further data, nobody is able to reproduce!

Hi Marcus thank you for contact.

This is the syntax i put in to [cookieDomain] version: 4.2.8

/\.(modellpilot\.eu)|(rc-modelpilot|rc-model-glider|rc-model-motor|rc-model-electric|rc-model-jet|rc-model-heli)\.com$/

If the FE-user is looked in, at www.modellpilot.eu or modellpilot.eu the jump to the next url for example rc-model-glider.com or www.rc-model-glider.com he isn't looked in.

Mr. Weiland checked it, too. The problem ist still find first time, since version 4.2.1.

What you trying to do is not working and this is expected behavior! It just how the web works.

$TYPO3_CONF_VARS[SYS][cookieDomain] is only useful with subdomains, not for complete different domains. The install tool explicitly mentions this:
"...login sessions will be shared across subdomains".

Think about it again. You trying this:
You are want to share cookies (sessions) between twitter.com and facebook.com. When accessing twitter.com, the browser will only send cookies created by twitter.com. When accessing facebook.com, the browser is not sending twitter.com cookies. You are trying and expecting that all of you browser cookies would be send to any domain, no matter which domain initially created them. This would be a major problem and would affect a user's privacy.

So there's no problem at all.

(Still wondering why jweiland-support wasn't able to tell you that.)

Reported issue is not a problem. Cookies aren't shared between completely different domains (!=subdomains). Hint: "Same Origin Policy"