504 time out when lang switch in frontend

Hi!
My site stalls when switching language in store frontend, just keeps loading until 504 timeout.
It's a fresh install 2302 with upgraded database from 1541.
VQmod installed.
SO clickboom theme (incl langauge patch for the theme+modules)
Swedish language pack.
All seems fine on english back+frontend, but as soon as I switch everything stalls. I can "unlock" it by going on a different browser and load default language (en), supposedly the cache/cookies are reset by doing this(?)

(Advanced user but new to multi language troubleshooting)

Does the language switcher work when using the default theme?

No, same problem on default theme.
The switcher in it self seems working, it's the page loading after that's the problem.