Apparently it was the Social Media Widget plugin that caused my issue.
In general, to troubleshoot such an issue with the admin page:
- Disable all plugins.
- Check that the problem has been solved, i.e., it is a plugin that causes this.
- Enable one plugin at a time and test, to find the culprit.
- Disable that plugin.
Otherwise, it might be your theme. Like plugins, themes can cause issues on the admin page.
- Switch to one of the default themes.
- Check that the problem has been solved.
- Now you know your theme is the culprit.
- Don’t use that theme.