fixed it by my own with a little hack (not exactly my prefered solution but it works for now)
my problem: it seems that with a AddOrUpdate(...)
i can only compare primitive types or enum types.
so i added an additional id called SeedId
to identify my entities.
so my code in the seed method is very simple now :
foreach (EventType eventtype in PqsDbContext.getStdEventTypes())
{
context.EventTypes.AddOrUpdate(et => et.SeedId, eventtype);
}