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
Thanks for this post. Was running in circles trying to figure out how to fix things after doing the same. All golden now.
For anyone else who finds this looking for help, Edit > Preferences > Advanced > Config Editor, confirm that you know what you’re doing if you see a screen asking you to, then enter “mail.cloud_files.accounts” and look for the incomplete entry. Should be at the bottom of the list you see. Right click the entry and hit Reset, then restart thunderbird. Assuming you got the right entry, your filelink configuration should be functional again.