Thanks for your quick response.
It happened when I used my mobile phone hotspot, as well as my wifi at home, both times the same error. The log was produced while logged in with my mobile phone hotspot.

That's very weird. I checked and reset the network properties on my iPad to default - same error. Afterwards, I set up eduroam network to connect to the wifi of our institution - the same network connection I am using on my PC and the same connection that works for my colleague's iPad - again the same error. The corresponding log nr is: D555467718

As a last resort I will try completely wiping and resetting the iPad later to try it again - otherwise, I don't know..

I cannot access it with the error message
"This connection is not private":
"This website may be impersonating "zoterofilestorage.s3.us-east-1.amazonaws.com" to steal your personal or financial information. You should go back to the previous page.

No, as I say, the certificate is valid — you shouldn't be clicking through a security warning, and now it might be harder to debug this. When seeking help here, please only do what we ask and not other things that we haven't said to do.

So there's definitely a problem with your connection, but since you accepted the invalid certificate, it might not be possible to view the certificate details. (iOS doesn't provide a reliable way of doing this.)

If you tap on, say, https://zotero-temp.s3.us-east-1.amazonaws.com/, do you get the same certificate error? If so, tap on "Show Details" and "view the certificate", tap "More Details", and provide a screenshot of the certificate details.

Yeah, something's very wrong on your device. That's a valid Amazon S3 certificate, but for some reason it's not being trusted by your device.

If you go to Settings → General → About → Certificate Trust Settings, what do you see?

If you go to Settings → General → VPN & Device Management, is there anything configured?

Do you get a certificate warning for the following URL?

https://valid.rootca1.demo.amazontrust.com/

Do you get a certificate error for https://zoterofilestorage.s3.amazonaws.com/ ?

I don't really know what to tell you. This would be breaking things across the web for you — images, other media files, even entire websites would be broken for you. Unfortunately Apple doesn't give full access to certificate debugging tools on iOS, so it's a bit harder to figure out exactly what's going on, but your device appears to be trusting Amazon's main root certificate, but for some reason not the Amazon S3 certificates. (Amazon S3 is the Amazon Web Services storage service used by much of the web.)

This is a problem with your iOS installation, not a problem with Zotero in any way, so I'm afraid you'll have to ask Apple for help here or try reinstalling iOS.

If you do try to ask Apple for help, I wouldn't mention Zotero at all. Just use a generic Amazon S3 URL like https://example.s3.amazonaws.com/ (As a reminder, an "Access Denied" page would be the correct result — a certificate warning would not.)

I have the exact same problem. iPad (6th gen). The Amazon root certificate is trusted, but the S3 certificate is not. Is there already a solution for this?
I would like to read and mark up documents on my iPad, but unfortunately nothing synchronizes because of this error

I will try rebooting my ipad this evening and try to reinstall the ios - let's see if that fixes it - otherwise, I will try to contact apples support.
I will keep you updated.

if you find a solution in the meantime @jan.schaffner, feel free to post it here!