Interesting issue....

[jamroco2000]

So I have an interesting one for you - please tell me if someone can replicate this on prestashop 1.5.0.17

1. Create a category
   
2. Then create a couple of sub-categories (at least 2)
   
3. Assign a product to one of those sub-categories except the first one on the list - make sure you set the default category to that same sub-category
   
4. Check your site - it should work fine
   
5. Now change it to the first one on the list - make sure the default category is also the same first sub-category
   
6. Now check your site - it blows up for me and gives a 500 error
   

I tried this is various ways - it seems that if I set the default category to be the first sub-category in the list of sub-categories under a category - it blows up. If you set the default category to anything else (i.e. the second sub-category - or the parent category) - it works fine

 

I can show an example if someone wants to see it - I thought it had to do with the custom theme I was using - but I disabled the theme and the problem still exists - so I'm not sure if its a module issue or what.