Spacemacs — themes won't load after adding to config layers

Question

I'm probably not understanding something basic...

Following the docs, I've added themes-megapack to dotspacemacs-configuration-layers '(...), but typing space f-e-R I'm told that themes-megapack has failed its test, and the themes won't load.

I tried removing all custom functions at the bottom of .spacemacs and it hasn't helped.

Here is my dotspacemacs-conficuration-layers:

 dotspacemacs-configuration-layers
 `(
   ...
   themes-megapack
   ...
 ;; ----------------------------------------------------------------
 ;; Example of useful layers you may want to use right away.
 ;; Uncomment some layer names and press <SPC f e R> (Vim style) or
 ;; <M-m f e R> (Emacs style) to install them.
 ;; ----------------------------------------------------------------
   //more packages, which load fine as long as they aren't themes//
 )

I've also tried adding themes-megapack in user-config, but it doesn't seems to help either.

Thanks for any help^^