TYPO3 Core

I ran into same behaviour recently.
This can happen if you have unnecessary new lines etc. in your configuration-files, e.g. before the opening php-Tag. Most likely in AdditionalConfiguration.php (or files you require in there).
Also check for control-characters that should not be there (e.g. ^M as a windows-control-character, even though your TYPO3 is running in on a unix-system).

Hannes Strangmeier wrote:

I ran into same behaviour recently.
This can happen if you have unnecessary new lines etc. in your configuration-files, e.g. before the opening php-Tag. Most likely in AdditionalConfiguration.php (or files you require in there).
Also check for control-characters that should not be there (e.g. ^M as a windows-control-character, even though your TYPO3 is running in on a unix-system).

Oh yes, wtf.
In my case it was a blank first line (befor however, I think this should not cause a corresponding error, or?!

In at least 7.6 and 8.7 this causes similar behaviours elsewhere, e.g. it would lead to a blank line as output of the scheduler when called on cli (otherwise it would just return an exit code 0).
I never really considered this a bug to be fair, since it's a misconfiguration in my point of view. This being said, in older versions i even had to modify an extension that also had such a blank line before the opening php-tag... but in most cases the realurl_conf.php had a blank line due to copy pasting from the example configs.