Ok. I figured this out. I will explain in case others run into this issue.

UNA uses php extension 'memcache' - there are two versions:

memcache and memcached. The second is more actively maintained and has newer features. Anyhow. I had memcached installed.

apt remove php8.2-memcached
apt install php8.2-memcache

Solved the issue, memcache now working.