SUMMARY
When you try to connect to any service with a URL including a password, if the password contains a question mark (?) the entire URL gets wrongly converted to the http protocol for some reason, even if it's already an http:// one.
If you try to escape the question mark a-la urlencode(), the connection works but then it sends the wrong password, because it's not decoded. So currently it's not possible to connect to any service using an URL with a password containing a question mark.

STEPS TO REPRODUCE
1. Try to connect to "ftp://test:pass?word@example.com"

OBSERVED RESULT
The URL gets converted to "http://ftp/test:pass/?word@example.com" for some reason and then Dolphin tries to open your web browser.

EXPECTED RESULT
An ftp connection is attempted with the correct password (including the "?").

SOFTWARE VERSIONS
(available in About System)
KDE Plasma Version: 5.12.7
KDE Frameworks Version: 5.47.0
Qt Version: 5.9.5