Have you reset the interface?
Ah I missed your comment. I’ll get back to you!
I’m quite positive I only left out irrelevant stuff such as firewall rules, wireguard config, adlist config, IP address allocations, static DNS records. I don’t see any of these being relevant for this issue.
If you know where the problem is, why dont you fix it yourself.
If you want others to address your issues then provide the information requested and stop being either arrogant or lazy or perhaps security afraid, ( Id prefer to think just security afraid ).
There is no harm in providing the config
/export file=anynameyouwish ( minus router serial number, any public WANIP information, keys, dhcp lease lists )
Paste this on terminal, on this exact order and do not omit { }
{
/interface bridge vlan
set [find where untagged=Trusted2.4,ether4,ether3,ether2] untagged=Trusted5,Trusted2.4,ether4,ether3,ether2
/interface bridge port
remove [find where interface=Trusted5]
add bridge=bridge frame-types=admit-only-untagged-and-priority-tagged interface=Trusted5 pvid=10
}
Too late 
Seeing as I’ve pasted it two times now there’s no need for this comment. I’ll continue this in a few hours; if you have any good ideas it would be great to hear them.
Haha see that now, great that you did. This issue is above my head, I only work in the kindergarten sandbox, but others here should help you solve it.
Yes. Unfortunately this doesn’t solve the problem. @erlinden resetting and reconfiguring the interface also doesn’t help.
Thank you both for the assist. I will be switching to a different WiFi stack soon, so I’ll just leave things as they are; 2.4GHz will suffice for now.
I had this exact same issue after upgrading my hap ax3. I have two older hap models that both upgraded fine but they have the older "wireless" rather than "wifi" settings.
Nothing I tried fixed the issue with 5GHz showing port is already a slave. So I spent a few hours on it last night and after deleting all the ports from the bridge setup and creating them again I still had the issue. I then edited the 5GHz wifi bridge port and assigned it to each of the other bridges in turn (I have three bridges for different vlans) and lo and behold it started working when I put it back on the one it is meant to be on.
I'm guessing somewhere a setting was corrupted or stuck and editing it, putting it on a different bridge and saving it - repeated for each of the bridges - seems to have sorted it for me.
Just thought I'd put this out here in case someone else tries this and it works for them too.
Generally it is is considered best practice to only have 1 single bridge, no matter how many VLANs you have ?
It happened to me when testing 7.23betaX. Nothing helped (except netinstall). 7.22.1 is OK. Do not upgrade to 7.23 or higher 
I hade the same issue: a wifi card that could not be added to a bridge "is aldeady a slave". then, following your suggestion, i added the wifi to another bridge, removed and added to the correct one. It worked like a charm.
This is a bug from mikrotik firmware
Same issue on an hAP ax3. It was already present on RouterOS 7.21.4 long-term and persisted after upgrading firmware to 7.23.2.
I tried cycling the bridge port through existing bridges and, after resetting and rebooting the Wi-Fi interface, attaching it to a completely new bridge. While disabled, the port was accepted. As soon as the 2.4 GHz radio entered running state, it became inactive with port is already slave.
I have disabled 2.4 GHz and am continuing with 5 GHz only...
i have the same issue - everything works fine on 7.20.8 long-term - after upgrade to 7.21.5 i got the same error on wifi2 "port is already slave"
Please post your config to give some advice, @ucaan.
/export file=anynameyoulike
Remove serial and any other private info, post as Preformatted text by using the </> button.
I have the same issue with my ax² and its still there in Version 7.24.2
Is there a solution or are we stuck at 7.20.8?
Have you seen my previous post?
The one which never got a response, you mean ? 
I know…