Hello, i spent much time to solve this Problem:
When i set up an Nextcloud-Filelink account and aborted the setup before entering password, the dropdown disappeared.
It is caused by an already stored lightning password for the same nextcloud account. A value in the about:config will be created for an invalid/non-existing account. This is a bug from the Add-On.
I reproduced the error in a Video: https://youtu.be/jznUN756dGE