I always use the protocol - it's one of the things I had to tweak about opera. But as I said: it is known that Chromium blocks ports. Here's the list, straight from the source code (and with line numbers):

namespace {

// The general list of blocked ports. Will be blocked unless a specific
// protocol overrides it. (Ex: ftp can use ports 20 and 21)
static const int kRestrictedPorts[] = {
1,      // tcpmux
7,      // echo
9,      // discard
11,     // systat
13,     // daytime
15,     // netstat
17,     // qotd
19,     // chargen
20,     // ftp data
21,     // ftp access
22,     // ssh
23,     // telnet
25,     // smtp
37,     // time
42,     // name
43,     // nicname
53,     // domain
77,     // priv-rjs
79,     // finger
87,     // ttylink
95,     // supdup
101,    // hostriame
102,    // iso-tsap
103,    // gppitnp
104,    // acr-nema
109,    // pop2
110,    // pop3
111,    // sunrpc
113,    // auth
115,    // sftp
117,    // uucp-path
119,    // nntp
123,    // NTP
135,    // loc-srv /epmap
139,    // netbios
143,    // imap2
179,    // BGP
389,    // ldap
465,    // smtp+ssl
512,    // print / exec
513,    // login
514,    // shell
515,    // printer
526,    // tempo
530,    // courier
531,    // chat
532,    // netnews
540,    // uucp
556,    // remotefs
563,    // nntp+ssl
587,    // stmp?
601,    // ??
636,    // ldap+ssl
993,    // ldap+ssl
995,    // pop3+ssl
2049, // nfs
3659, // apple-sasl / PasswordServer
4045, // lockd
6000, // X11
6665, // Alternate IRC [Apple addition]
6666, // Alternate IRC [Apple addition]
6667, // Standard IRC [Apple addition]
6668, // Alternate IRC [Apple addition]
6669, // Alternate IRC [Apple addition]
0xFFFF, // Used to block all invalid port numbers (see
        // third_party/WebKit/Source/platform/weborigin/KURL.cpp,
        // KURL::port())

My astonishment is both at the fact that no one seems to care about this - if IE would do this, it'd be a a front-page scandal on every tech site - and at the apparent lack of solution either.

From what I found on the internet, there used to be a setting called 'permitted ports'in earlier builds', but it seems to have disappeared. Apparently I'll have to get the source somewhere (if possible), edit - rather: clear- the list and compile my own version. Which obviously isn't very user-friendly.

After nearly 20 years, I decided to give Opera a try again. I'm pretty happy with it after some tweaking, except for
one thing: it blocks https access to specific ports and I don't find a way to override it. I have devices running web
servers at exotic ports like :1234 and opera simplyh claims they don't exist.

Make sure to specify https:// and then the address / hostname with the portnumber instead of simply specifying the name and number. If I use mysite.org:4443 then I get the same error as you, but when I use https://mysite.org:4443 it simply works as expected for me. Maybe that could help?

I'm surprised that it's so hard to find information about this. This doesn't seem to be such an exotic question.