KPIs not loading in campaign without any subcampaigns when "first contributions" is active

[BenediktMagnus]

When the KPI "first contributions" is active the KPIs in the campaign view do not load and stay empty for campaigns without any subcampaigns.
In campaigns (either root or a subcampaign themselves) with subcampaigns the KPIs load normally.

The result of the XHR loading the KPIs only includes this (even if Civi's debug mode is active):
{"error_message":"Unknown error.","is_error":1}

[bkaless]

I found the same issues in campaigns with subcampaigns as well.