TYPO3 Core

Could you try as workaround the following steps:

1. Set missing flag for all files in your local storage(s)
   UPDATE sys_file SET missing = 1 WHERE storage = 1;
2. Run the "File Abstraction Layer: Update storage index (scheduler)" for all your local storages

The indexer will find the missing files and re-indexes them before the missing flag is reset.
In the indexer has some basic functionality to find image dimensions for files in local storages so when re-indexed the width and height are fetched and set again for the images.

While Running the task I get:

Execution of task "File Abstraction Layer: Update storage index (scheduler)" failed with the following message: Task failed to execute successfully.
Class: TYPO3\CMS\Scheduler\Task\FileStorageIndexingTask, UID: 1

Nothing written in any log file.

Debugged it to show the exception:

RecursiveDirectoryIterator::__construct(/.../typo3-master/fileadmin/a): failed to open dir: Permission denied

Uncool the indexer falls about the same stuff we fixed for the tree.

BTW. We shouldn't catch the Exception, the Scheduler handles Exception themselves. Removing the catch would bring following message:

Execution of task "File Abstraction Layer: Update storage index (scheduler)" failed with the following message: RecursiveDirectoryIterator::__construct(/.../typo3-master/fileadmin/a): failed to open dir: Permission denied

Not nice but it tells more to the user.

Alexander, perhaps you could make a new issue out of your last comment. Issue still exists in 6.2.6, leading to error message without any details in FAL indexer and no info whatsoever in logs. Passing the original exception should be somewhat comparatively easy to fix, I suppose.