TYPO3 Core

TYPO3 9.5.8
PHP 7.2.10

Link wizard produces a 500 error when calling for example if I want to add a picture in the text & media element in the BE - (this popup / link wizard I mean) - error is present as soon as several 100 files are in the fileadmin.

Specific case:
TYPO3 update from 6 to 9 - customer has unfortunately not really kept order, so now there are 1054 files directly under fileadmin / user_upload / ...

More files are nested in folders, but that does not seem to be a problem, because as soon as the files are directly under user_upload are drastically reduced or be taken out completely, the wizard works again.

-------- apache:
[error] Premature end of script headers: index.php, referer: {...}/typo3/index.php?route=%2Fmain&token=9e1d044c1d708835d7973603a89f0b1265f33e24

-------- browser console: {...}/typo3/index.php?route=%2Fwizard%2Frecord%2Fbrowse&token=8624191c0a714ddc89baac4977e80254e4037743&mode=file&bparams=|||gif,jpg,jpeg,bmp,png,pdf,svg,ai,mp3,wav,mp4,ogg,flac,opus,webm,youtube,vimeo|data-1-tt_content-1504-assets-sys_file_reference|inline.checkUniqueElement||inline.importElement 500 (Internal Server Error)