[horde] "502 Bad Gateway" and 'Undefined array key "secret_key"' on fresh Horde 6 install

[Sebastian Arcus]

I have a few Horde 5.x installs and I decided to try out Horde 6.x and 
get a bit familiar with it. I've followed the standard instructions from 
the url below for installing a fresh copy using composer.

https://wiki.horde.org/Doc/Dev/H6/Installation/InstallBase

I've reached the point where I'm supposed to access the config URL - but 
getting "502 Bad gateway" error in the browser. There doesn't seem to be 
any error message in the Lighttpd logs with debug turned on. The only 
error message I can find is in syslog - which is repeated every time I 
am trying to access any Horde url:

Jun 22 17:36:08 horde6 HORDE: PHP ERROR: Undefined array key 
"secret_key" [pid 2891 on line 21 of 
"/srv/lighttpd/horde6.mdrinteriors.co.uk/vendor/horde/core/lib/Horde/Core/Factory/Secret/Cbc.php"]

I am on:
Slackware 64
PHP 8.4.82
Lighttpd 1.4.82

Am I doing something silly - or maybe Lighttpd is not supported any more 
on Horde 6.x? Any ideas much appreciated