Some of you might get an unmeaning error message on setting up filelink that there was an error. For me i figured it out after some (acutally a lot of) googling:
When using a self-signed certificate you will get this error message. You have to add the certificate to the exceptions. Go to Setttings --> Advanced --> Certificates --> Manage Certificates --> Server and add your server there. Note that your local IP https://192.168.178.xyz and https://my-nextcloud.no-ip.org are not the same. Add the one you will use for filelink. Only the latter makes sense unless you’re only sending emails within your private network.
May I suggest the authors of this marvellous plugin to add this note in the installation instructions or even improve the plugin that you can grant the exception within the setup process?