Icon doesn't display

Steps to reproduce

1. update nextcloud to 18.0.2 (from version 15)
2. update "External Sites" app
3. log in to nextcloud — the link to the the external site shows, but no icon
4. open "External Sites" admin settings — any icon that is added, shows a broken img

Expected behaviour

The icon should display properly in the topbar's general view.
Adding an icon in the "External sites" admin options should show non broken img sources.
It should be possible to remove added icons using the little trash button.

Actual behaviour

There is no icon in the topbar (but the text of the external site shows when hovering over the button).
All icons in the "External Sites" admin settings show a broken source.
Clicking on the little trash icon to remove a previously uploaded icon doesn't do anything.

Server configuration

Operating system: yunohost 3.7.1.1 (Debian 9)
Web server: ?
Database: mysql 10.1.44
PHP version: 7.3.16
Nextcloud version: 18.0.2

Where did you install Nextcloud from: yunohost app store see here for details

App enabled

Accessibility 1.4.0
Activity 2.11.0
AppOrder 0.9.0
Auditing / Logging 1.8.0
Calendar 2.0.3
Circles 0.18.4
Collaborative tags 1.8.0
Comments 1.8.0
Contacts 3.2.0
Deleted files 1.8.0
Draw.io 0.9.5
External sites 3.5.0
External storage support 1.9.0
External user authentication 0.9.1
Federation 1.8.0
File sharing 1.10.1
First run wizard 2.7.0
LDAP user and group backend 1.8.0
Log Reader 2.3.0
Markdown Editor 2.2.0
Monitoring 1.8.0
Music 0.13.2
News 14.1.3
Nextcloud announcements 1.7.0
Notifications 2.6.0
ONLYOFFICE 4.1.4
Ownpad 0.6.14
Password policy 1.8.0
PDF viewer 1.7.0
Photos 1.0.0
Privacy 1.2.0
Recommendations 0.6.0
Right click 0.15.2
Share by mail 1.8.0
Support 1.1.0
Text 2.0.0
Theming 1.9.0
Update notification 1.8.0
Usage survey 1.6.0
Versions 1.11.0
Video player 1.7.0

Client configuration

Browser: firefox 75
Operating system: ubuntu 18

Start by reproducing the missing topbar and broken admin icon behavior with External Sites 3.5.0 on Nextcloud 18.0.2, then inspect the app's icon and trash-button handling. Done means icons display in the topbar and admin settings, and previously uploaded icons can be removed.