Hello,
I’m new to everything here (nextcloud, forum) and I hope I post in the right place.
Here is the issue I face with: when I try to link a file, although it’s uploaded in my nextcloud space, the link doesn’t appear into the message and a pop up says that the upload is impossible.
How can I fix it (if I can)?
I’m using ubuntu 18.04 and I create a webdav link without any problem.
Thank you in advance.
I use this and its ok for me : https://addons.thunderbird.net/fr/thunderbird/addon/filelink-nextcloud-owncloud/
I use it too 
Maybe you need to use an app password ?