id=undefined when changing template on new resource (resulting in 404)

When I change the template of a new, not yet saved resource and accept the alert 'are you sure you want to change the template..', something goes wrong with the redirect url. It looks like this:

http://www.mywebsite.nl/manager/?a=resource/create&id=undefined&reload=5808d46f9d5895.59181424&class_key=modDocument

And I get a 404 response.

I'm not sure where to look for a solution.

Revolution 2.5.1
- ContentBlocks 1.5.0
- Collections 3.5.0
- Redactor 2.2.0
- and some more, but I think these are the ones that could have something to do with it, although deactivating these plugins doesn't fix it.

PS I did find an issue on github from 2012 describing kind of the same problem, but I checked and the suggested fix was already implemented in my installation (not so surprising since it was from 2012 ;-)

Hoping someone has had this problem before and can help me!

Are you logged in with full administrator rights?

You marked id=undefinded as bold: The id is not created as long your document isn't saved, so this is not uncommon ..

What happens when you save the document before and then change the template?

Yes, I'm logged in with full administrator rights.
I marked that part bold because when I change 'undefined' to for example 0 (zero) the manager page loads normally.
But I see now that on other sites it says undefined as well (but I just don't get the 404).

When I save first and then change the template it's working normally, so maybe this is not such a big issue after all. I'll just have to explain that to my client. ...Should have tried that before posting :-D

Thanks for your help!