I have today reported the following to DrayTek directly, but post here as a heads up to anyone with AP 903 units...

I upgraded the firmware on my AP 903 from 1.3.6 to 1.3.7, only to discover the following issues:

1. If I change the 2.4GHz and/or 5GHz wireless security from WPA2/PSK to WPA3/WPA2 Personal, then WPA3-capable devices can no longer connect because of ‘invalid passphrase’: If I re-enter the (same) passphrase on the device it works, but then the device can no longer connect to our 2862ac router, because the passphrase is no longer accepted by the 2862 (although it is configured with the same SSIDs and (WPA2/PSK) keys.
2. Possibly because of the above bug, if the AP 903 is configured for WPA3/WPA2 Personal, devices can no longer roam between the 2862ac (which doesn’t offer WPA3) and the AP 903.
3. On firmware version 1.3.7, the AP 903’s Management page (in the System Maintenance section) of the GUI is completely blank.

On reverting to firmware 1.3.6, these bugs disappear. I suggest DrayTek withdraw the 1.3.7 firmware until they can make it work.

I haven't tried WPA3, but on my AP903s with 1.3.7 the Management page is fine (Chrome on W10).

cocospm wrote:
3. On firmware version 1.3.7, the AP 903’s Management page (in the System Maintenance section) of the GUI is completely blank.

I can confirm that one (haven't ventured into the world of WPA3 yet). "View Source" reveals a lot of valid looking HTML - I've not tried to work out why it doesn't actually display anything... (Browser is this new, fancy-dancy Microsoft Edge thing)

My management page is fine in Edge as well as Chrome.....

It turns out, that I can access "System Maintenance >> Management", if I access the Vigor AP903 using its IP address, or the fully-qualified DNS name...

Normally, I use "HTTP://VigorAP903/" where VigorAP903 is defined in my local DNS. This still works for every page (I've tried), except "Management".

VigorAP903 - fails (gives a blank frame)
VigorAP903.home.mydomainname.co.uk - works
192.168.100.251 - works

Ah - I usually connect to mine via "External devices" on the router, which is IP based.