Hello, Nikolai!
Sorry to hear that.
There must be some conflict with the plugins you have activated and the theme.
Please check THIS video guide to manage to log in to your WordPress dashboard,
Then deactivate ALL active plugins and install the theme again.
After that, to find which plugin causes the error, activate them one by one until the problem occurs again, so you will know the problematic plugin, then repeat first procedure, and keep the plugin deactivated.
After that , let us know which plugin causes the conflict so we can try to fix that.
If you need any help with the procedure mentioned above, please contact again.
Best Regards