Customer clicks on 'Proceed to Checkout' button, gets redir to crt with blk err msg

[netsoftservices]

Magento ver. 1.9.0.1
Default Template (modified)

php version: 5.5.30

I have an inherited site that I am cleaning up. I have all but one item fixed on the site...

I have a site that works great except for when they have a promotion going on. When a promotion code is used, or an in-cart promotion is set active and a person clicks/taps on the 'Proceed to Checkout' button, it redirects back with a blank error message in a red error box. If I remove the promotion or remove the promotion code from the cart, all works fine and the user is sent to the checkout page. I can reproduce this 100% on my development environment as well.

I have found that that error message is coming from Mage_Core_Block_Messages. When I var_dump it, I see that it is truly blank. It is a message of type: object(Mage_Core_Model_Message_Error)#88

I have all of the debug options on and have checked all the log files. Nothing shows what this error is. Could someone please give me any more debug tools so that I can find out why this is highjacking the user from getting to the checkout page? They are using onepagecheckout.

I have used SetEnv MAGE_IS_DEVELOPER_MODE "true" and also set all the other debug flags.

I have googled this for weeks now. I am at the end of my knowledge here. Just needing some better explanation of what might be going on here to stop a customer from being able to get to the checkout page.

One other item of note, the client needs certain users to not pay tax, so they have a checkbox set for those. If I unselect this checkbox and save the user, they can get through to the checkout.

Thank you for any info. I am happy to add more detail here. Just need to know what other info you need.

Doug

[ACheckout]

Dear @netsoftservices,

I think you are using Minumum Order Amount feature in magento. You can check this config in your backend->System->Configuration->Sales->Minumum Order Amount. If your order have order lessthan Minimum Amount, magento system will redirect from checkout page to cart page and show message in configure of  Error to Show in Shopping Cart.

[netsoftservices]

Dear @ACheckout

Thank you for this. You rock!!

I see now that they had Min Order Amount set to YES, but no value and no messages set up. I set the min to $5 and added messages/error text. I now get the minimum amount error text when trying to go to checkout. Is there anywhere else that this min amount could be being set? The cart value is $138 and the min is $5, but I am still getting the min amount error and no redirect to the checkout. Any other thoughts. I know that it is soooo close!! As a reminder. It only happens when a coupon or shopping cart price rules come into play.

Thanks again for your insight!!

[netsoftservices]

Also, just by setting this to NO makes it so they can get to checkout, but the client wants a minimum cart amount ($19). Really odd issue here. I don't know where else the min cart amount could be set.