suddenly
(i did redis flushall before.. for another issue, this matters?)

anyway, suddenly, i am getting 502 bad gateway from nginx
then rebooted
then, there's gitlab error 502.. taking too much to respond error.
then, i did

sudo gitlab-ctl reconfigure

like:
https://gitlab.com/gitlab-org/omnibus-gitlab/-/issues/4893#note_254058178

then, it is fixed.