Error when adding coupon to order in sales->order

Hello!

I am running OC 1.5.6.1 from a fresh install. I have a tax extension to display order totals incl VAT: https://www.opencart.com/index.php?rout ... _id=431297

I need some help tracking down an issue that keeps me from adding a coupon to an order in administration->Sales->Orders.

When I bring up the order it looks okay. Then I enter the coupon code and hit 'update total': It's obvious something goes wrong, the shipping fee is gone and the coupon code line looks strange. I get an error message that says:
Also the shipping and payment method drop downs are empty after updating totals.

I have tried finding clues in the logs but unfortunately no entries there. I would really appreciate some advice on how to track this error down.

I have most likely left out vital information so please, do not hesitate to ask me.

You'd need to contact the maker of that extension. Default opencart does not give such an error. If you paid for the extension then you should receive good customer service.