Please help us help you and fill where relevant:
Your LimeSurvey version: LimeSurvey Community Edition Version 6.4.1
Own server
Survey theme/template: N/A
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I am very new to LimeSurvey. I am trying to experiment with self-hosting and so far it seems to work really well. My only problem is that I can't seem to import a survey at all. I get the following error:

"An error occurred uploading your file. This may be caused by incorrect permissions for the application /tmp folder. "

I checked permissions and there's no issue that I can see. I even made it world writeable just to see if that clears it up, but it still reports this issue.

I can't find any way to get any details about what the actual issue is from the server. LimeSurvey doesn't seem to keep any logs, and the error message doesn't seem to actually interrogate whatever library or framework it's using to upload and store files as to the actual nature of the problem.

I am using the NixOS module. I could try a more "vanilla" installation, but even if that clears it up, it would still be helpful to know how to get useful diagnostic information from LimeSurvey.

I tried setting debug to 2 in the config and saw nothing change. I don't know if it's the result of the NixOS module or not. Googling suggested maybe this is a size issue, but the survey file comes well under 2 MiB so it can't be that.

You can activate debug mode : manual.limesurvey.org/Debug_mode

Think it's here : mynixos.com/nixpkgs/option/services.limesurvey.config

(Arg, if i don't make error : you need to construct whole OS again for NixOS)