guix package: error: build failed: some substitutes for the outputs of derivation hash.drv failed (usually happens due to networking issues); try `--fallback' to build derivation from source

[znavko]

Is there any description of mechanism of updating and package installation.
I think guix downloads sources and then build them. I think there is a cache od downloaded archives.
But what exactly do here?

# guix package -u
updating substitutes from 'https://mirror.hydra.gnu.org'... 100.0%
substitute: updating substitutes from 'https://mirror.hydra.gnu.org'... 100.0%
guix substitute: error: download from 'https://mirror.hydra.gnu.org/guix/nar/gzip/yq82kwc1id78087idl8j4ywmy22b4v80-gtksourceview-3.24.8' failed: 504, "Gateway Time-out"
substitution of /gnu/store/yq82kwc1id78087idl8j4ywmy22b4v80-gtksourceview-3.24.8 failed
killing process 32540
guix package: error: build failed: some substitutes for the outputs of derivation `/gnu/store/8z50cls0f27yimw3qyn90ck2fwndsic9-gtksourceview-3.24.8.drv' failed (usually happens due to networking issues); try `--fallback' to build derivation from source 

# guix package --fallback
# guix package -u
guix substitute: error: download from 'https://mirror.hydra.gnu.org/guix/nar/gzip/yq82kwc1id78087idl8j4ywmy22b4v80-gtksourceview-3.24.8' failed: 504, "Gateway Time-out"
substitution of /gnu/store/yq82kwc1id78087idl8j4ywmy22b4v80-gtksourceview-3.24.8 failed
killing process 32581
guix package: error: build failed: some substitutes for the outputs of derivation `/gnu/store/8z50cls0f27yimw3qyn90ck2fwndsic9-gtksourceview-3.24.8.drv' failed (usually happens due to networking issues); try `--fallback' to build derivation from source 

# guix package -u --fallback
guix package: warning: upgrade regexp '--fallback' looks like a command-line option
guix package: warning: is this intended?


Do I need to delete cache files?