Please help us help you and fill where relevant:
Your LimeSurvey version:  LimeSurvey Community Edition   Version 6.1.8+230717 
Own server or LimeSurvey hosting: Own server (nas synology)
Survey theme/template: Sea Green, standard unmodified administration theme
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Using the Ressources tool, KCFinder is almost impossible for me as I usually have this "erreur inconnue" (unknown error) in the screen.
I checked the rights, the owner on the server side, all seems to be fine.
Could someone help me to figure out, why KCfinder cannot show and upload the files properly ? 
As my installation is a POC (proof of concept), I only have two unpublished surveys and the same happen on both.
Steps :
1. open a survey
2. click on the "ressources" menu on the left side
3. select the "images" option on the left-top of KCFinder windows
4. try to visualize or to upload new (see attached)

Solution :
Try on different browsers, results might be slightly different, which makes the story even more confusing.
++ Grants correct rights : in my case, syno web group + user &  744 made is working fine .

I just updated my LS 6 to LS 6.1.8 to try to reproduce your issue. However, no issues encountered here. So it doesn't seem to be an issue with 6.1.8 (at least not one that affects everyone):
 

My first suspect would be folder permissions. You said you checked the permissions, but let's give it another try.

What are your permissions on the upload folder and the folders within it? Check specifically the ones for the survey you are testing? Do they inherit the permissions of the higher level folders or not? (/upload/surveys/)

Tks for the reply and, yes, it must be something linked to the rights.
As the readme file found in the original folder, I chmod -R the full folder with 755 rights.
At the moment, the workaround I applied is to upload the images directly in the folder as I am mainly using the html tag to use the images and it works.
The case is not solved for me but as this work around works, shall I close the subject ?
Again, thank you.

The case is not solved for me but as this work around works, shall I close the subject ?

No, leave it as it is. Maybe someone else has another idea of what it could be.

here is what the manual says regarding folder permission:
www.limesurvey.org/manual/Installation_-...irectory_permissions

So 755 should be fine.

Hello,

I had the same issue with a server running on a specific port (8084), and it was resolved by deploying the zip file available at the URL : https:// discuss phplist org/t/kcfinder-with-niginx/8223/7

Although browsers react differently, in the end it was the chmod rights assigned that made the difference.
SSH + chmod in sudo mode solved the problem.
Thanks for your help and answers

bquiller wrote:

Hello,

I had the same issue with a server running on a specific port (8084), and it was resolved by deploying the zip file available at the URL : https:// discuss phplist org/t/kcfinder-with-niginx/8223/7

Can you report the issue of you still reproduce in last 5.X or master ?
We must allow any port

OK, thank you : i made the issue 19240

Thank you !
bugs.limesurvey.org/view.php?id=19240

Not tested currently (to be confirmed)