Endess loop of empty error messages

Expected behaviour

If an error occurs, show meaningful text in error popup.

Actual behaviour

Until image upload is activated (on a freshly installed app), app behaves normally. File upload works.
When image upload is activated, an empty popup / overlay appears every 2 seconds. The popup contains just a (x) in the upper-right corner.
After closing the popup about 10 times, nextcloud app asks for login credentials.
Popups continue to appear if image upload is deactivated, rendering the app unusable.
No images are uploaded.
No errors are logged to by the docker container.

Steps to reproduce

Reinstall app. Activate image uploading.
Run Docker image nextcloud:17 on minikube.
Set ingress metadata: annotations: nginx.ingress.kubernetes.io/proxy-body-size: "100m" in order to avoid HTTP error 413 for image uploads (which the app would actually report).

iOS version

13.1.2

App version

2.24.4

Server configuration

Operating system:
debian buster, running minikube 1.4.0, with docker-ce 19.03.4

Web server:
apache in docker image nextcloud:17

Database:
sqlite

PHP version:
php bundled with docker image nextcloud:17

Nextcloud version: (see Nextcloud admin page)
docker image nextcloud:17

Start by reproducing the issue on iOS 13.1.2 with app version 2.24.4, Nextcloud 17, and image uploading enabled. Trace the image-upload failure and the empty error-popup flow; it is done when failures show meaningful text, repeated popups stop, and image uploads either succeed or report an actionable error.