500 Internal Error on-click of Admin- Module Loader in SugarCRM Version 10.1

Error 500 reasons can mostly be identified by checking the php error log.

In Sugar cloud based systems you can download it in admin - cloud insights  

In self hosted systems check your php.ini for the path of the file.

ModuleInstall/PackageManager/PackageManager.php L:935

list($manifest, ) = $this->requireManifestFile($target_manifest);

if you comment the line, then $manifest will be null, but the ModuleLoader is at least usable

By the way - in the old version 9.0.1 - the $manifest variable was not defined as well.
If the variable $base_upgrade_dir (Line 910) would not be hardcoded, but defined by config - it could works as well.