@Periosa There are currently some issues related to isolated-vm.
We are actively working on fixing this issue.
You can join our slack to get up to date messages about the current state (you can join the #ivm channel to see details of what's happening).
Please beware that the isolated-vm feature is still in the experimental fase and might cause issues like you described earlier.
You can also subscribe to this topic to receive updates: https://screeps.com/forum/topic/2133/changelog-2018-03-05/14