ob run terminates with an error

[tkx68]

After freshly cloning this repo I get the following error:

>>> git clone https://github.com/reflex-frp/reflex-examples.git
Cloning into 'reflex-examples'...
remote: Enumerating objects: 1637, done.
remote: Total 1637 (delta 0), reused 0 (delta 0), pack-reused 1637
Receiving objects: 100% (1637/1637), 2.14 MiB | 875.00 KiB/s, done.
Resolving deltas: 100% (681/681), done.
>>> cd reflex-examples 
>>> ob run 
./.obelisk/impl: command not cached, building ...
DONE Built on ./.obelisk/impl [command]
Process exited with code 1; 'nix' 'eval' '-f' '.' '(let a = import ./. {}; in toString (a.reflex.nixpkgs.lib.isDerivation a.passthru.staticFilesImpure))' '--raw'

What should I do?

When I did this for the first time there where many build actions that were performed successfully before ob run finally failed. Nut I cannot reproduce the successful actions and messages. I hope these are not relevant?