DeepDeck Installer v0.5 gives error when changing anything on the embedded web site.

[axello]

I got the webserver up & running on 192.168.4.1.

I tried entering my wifi network, but was greeted with an 'Error Code: 0'
'Message: Http failure response for http://null/api/connect: 0 Unknown Error'

I tried this 3 times. UI Version V0.5.4

All the time when the web page was shown, the OLED display showed "Waiting for connection…"
Clicking KEYMAP and MACROS gave the same error.

These errors occurred both in Safari and Chrome.

Update

I should add I am only connected through the website. The dot after 'Test Connection' is red, and clicking the button does not change anything. The Ahuyama refuses to connect to the existing Bluetooth connection.
Removing the Bluetooth connection and pairing again does also not solve the issue: No matter how many times I 'Connect', it never creates a stable connection, something it used to do in the previous iteration of the firmware.