It seems that Thunar does not properly handle URLs to remote resources
that use an IPv6 address for the host.
When I try to enter an FTP address like ftp://[2001:db8::1234]/ for the
location, I get an error saying, 'Error resolving "[2001": Name or
service not known.'
Similarly, when I enter an SMB address with a username like
smb://example@[2001:db8::1234]/ I get an authentication prompt saying,
'Enter password for "[2001".'
Both of these suggest that the URLs are not being parsed correctly.
A workaround is to mount such resources using the "gio mount" command
and browse them in Thunar as normal. This suggests that the wrong URL
parsing is happening in Thunar and not GIO or GVFS.
I also get generic "connection failed" and similar errors with other
kinds of URLs, like smb: without a username, but I suspect the cause is
the same.