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You should check the configured PHP "error_log" for more details. Perhaps PHP is missing a value for "date.timezone"? (The directive is commented by default inside "php.ini" and needs editing and uncommenting.)

If typing your username and password inside the browser's login prompt doesn't let you proceed, it means your web server is not configured properly and it does not pass the HTTP Basic Authentication data to PHP.

What web server are you using? Is it configured by you or are you using a web hosting service?

Indeed seems to be a bug in the current FileRun version, causing languages with names containing more than a single word to not work properly. This will be fixed with the upcoming update. In the meantime, a workaround is to rename the language file to remove the space character (for example, rename "system/data/languages/chinese traditional.txt" to "chinesetraditional.txt").

FileRun currently doesn't have any kind of support for RTL.

P.S. I am changing this to a feature request, so other interested users can upvote.

Can you access "https://myserver/FileRun/dav.php/" with your FileRun credentials?

Sounds like the permissions of the folder "/var/filedir/" do not allow PHP to make changes. The recommended action would be to check the folder's permissions and change accordingly.