Few questions: what version of PHP are you using?
There was an unexpected error when the plugin tried to use your credentials and connect the WebDAV server. Go to the Support tab and click the Check PHP Setup. Make sure the essential PHP libraries are enabled (green, ie. not with red).
If that’s OK then probably you are unable to use that URL from your server. Usually I use the cadaver command-line utility to check whether the WebDAV is accessible or not. Look at the following example:
cadaver https://www.crushftp.com/demo/
user: demo
password :demo
dav:/demo/> ls
If that works from your web server (note: this must be test at the web server level not on your local station) then there is something odd with the PHP libraries you use.
Btw: does your WebDAV connection use other port than port 80? If that’s the case then make sure your URL has the following format: https://hostname%5B:port%5D/path
