Magento 1.9.2.4 Product Import stuck on "Check data"

[BrandCommerce]

I have installed a fresh Magento 1.9.2.4.
I imported all my products orders etc, with a 3rd party plugin, which worked fine.
Now I am trying to use the Default Magento CSV Import for Product updates, but when I click on "Check data" the "Please Wait..." symbol does not go away and nothing happens.

I also get this error in Chrome Console:

form.js:108
POST http://www.mydomain.com/index.php/admin/import/validate/key/63e9b4ee2e23be5516cfaf671540afaf/?form_k... 500 (Internal Server Error)
_submit @ form.js:108varien
Form.postToFrame @ (index):550
onclick @ (index):627

I've tested my csv file on other magento installations and it works fine.

[BrandCommerce · Accepted solution]

SOLVED!  Turned out to be a corrupt import.php file.

Replaced app/code/core/Mage with a healthy copy and all worked again.

Very frustrating considering it was a fresh installation.

[CNCPros]

Spoiler

Please help!!! I am not an experienced programmer. When I try to do a product import and click "Check Data" it gets hung up on "Please Wait". We have a stage site and it works just fine on the stage site but when I try to import on the live site it gets stuck. I don't know what to do.