Your comments

FileRun makes no difference between the type of files you are uploading, or their contents. So there is no point in testing with a specific file.

If you have problems only with a specific type of file or its contents, the problem is not from FileRun. It could be from an antivirus or firewall, or something like a misconfigured "mod_security" Apache module, you might have in your infrastructure.

Previews open in a full-screen inner-popup. This popup has a left-pointing arrow in the top-left corner. You use that to close the popup and return to the list of files (from search results or folder).

After pressing OK, is there anything displayed? As written above, the PHP error log might include some useful troubleshooting information.

Also, in your PHP installation, if you get an option of installing the extension "nd_pdo_mysql", try to do so (and after, if still no luck, try to remove the "pdf_mysql").

If you applied the patch but the option is still missing, installing PHP's "mysqlnd" extension might help:

sudo apt-get install php-mysqlnd

What do you see when you browse inside?

I see an error message from your Lightspeed server in there, not from PHP or FileRun.

When you get this error, you can download files fine?

$config['search']['limit_file_size'] = 10485760;

Add the above inside the config file (see how to create it) and adjust value accordingly.

Tika might need permissions to write to a temporary folder.