I found the cause, it was a misconfiguration of my web server which prevented accessing images from the source wiki.

This can happen in the wild, if an image file is moved or permissions change we'll show this terribly unformatted and low-level error message.

Steps to reproduce:

• Remove or chmod a-r the image file on the source wiki, under the images/ directory.
• Export this file to the target wiki using the FileExporter button.
• Submit the import.

Confirmed, this is still a bug. The steps to reproduce above are correct. Additionally, when the AbuseFilter extension is enabled I get a fatal crash and stack trace, but this looks like a separate issue.

I'll repurpose this task to do the fixing.

Change 793481 had a related patch set uploaded (by Thiemo Kreuz (WMDE); author: Thiemo Kreuz (WMDE)):

[mediawiki/extensions/FileImporter@master] Log and show reasons when commit operations fail

https://gerrit.wikimedia.org/r/793481

I just noticed that this here might be the same issue I described in T308753 and tried to fix in https://gerrit.wikimedia.org/r/793481. It's probably worth doing further tests with the proposed patch already applied.

Change 969315 had a related patch set uploaded (by WMDE-Fisch; author: WMDE-Fisch):

[mediawiki/extensions/FileImporter@master] Show readable message from HttpRequestException errors

https://gerrit.wikimedia.org/r/969315

https://gerrit.wikimedia.org/r/969315

With the patch the error looks like this now:

Change 969315 merged by jenkins-bot:

[mediawiki/extensions/FileImporter@master] Show readable message from HttpRequestException errors

https://gerrit.wikimedia.org/r/969315

Change 793481 merged by jenkins-bot:

[mediawiki/extensions/FileImporter@master] Log and show reasons when commit operations fail

https://gerrit.wikimedia.org/r/793481