I think I fixed it - all I had to do was a remap; not sure how that wrong URL was there in the first place, as the error appeared after a rebuild that was not done by me.
Anyhow, this is what I did to fix this:
> ./launcher enter app
> discourse remap //theme-javascripts /theme-javascripts
> Rewriting all occurrences of //theme-javascripts to /theme-javascripts
> THIS TASK WILL REWRITE DATA, ARE YOU SURE (type YES)
> YES
> theme_fields=4
> Done
Hope this helps somebody else in the future.