This post is just for your information that I already sent this bug to grommunio GmbH via their ticketing system.
The arrows near the folder names which indicate that the folders are expandible/collapsible are hidden behind the folder content container if the folder names are too long (at least one folder needs to be long for that). Tested in Firefox and Chromium-based browsers.
As uploading of animated files is not allowed here (which makes it less comfortable) 😜 I uploaded the file here:
https://next.sweetba.se/s/8CcHZAZAzCaF95p