i think you are going to have a tough time with this module. they have not updated it in years. they are not releasing it through the normal zen-cart plugin review process, and there is no support of it here at the ZC forum as far as i can tell...

based on the little digging that i did, i would use only the payment window code and get rid of the flexwin code. the payment window code is the newest.

the dist directory are zip files of the releases. the latest release is:

https://github.com/DIBS-Payment-Serv...1_DPW4.1.3.zip

you can remove all of the other things, download this zip file, extract the files from within the zip, and then install those as according to whatever documentation you have. if you are satisfied that you did everything proper with copying the contents of:

Zen-Cart-DIBS\Zen-Cart-master\src\DIBS Payment Window\

you can safely ignore the files in the dist directory. but definitely get rid of the flexwin ones.

with regards to your specific error, i do not have anywhere solid to point you. it looks like something maybe redirecting you to the checkout_payment page, but it is pointing to the wrong spot. do you know your directory structure on your server? does the folder:

/zen-cart-v1.5.5e-03082017/

exist on your server? the problem might be here:

includes/modules/payment/dibspw.php:104
includes/modules/payment/dibspw.php:162

i would talk to the DIBs people as they will really be the ones to help with this problem.

good luck!