Hi, we have the same issue. We tried like 1000 things but nothing worked. We contacted the customer service. If there is any update i will forward it to you.

If you take a look in this guide you can see different ways of making reservations.

https://community.eurail.com/train-connections-reservations-47/how-to-get-reservations-105

The most common reasons why it failed are:

1.THIS REServ is simply not possible at all online

2.there are some hickups in processing 

It is indeed always better to try via sites of railways concerned. Indeed it is not user-friendly to send you in the woods like that, but as for now it is what it is.

Polish people are famous for being able to discover the other ways and ALTernatives to avoid the extra cost.

Hello! This used to be a system error due to the heavy traffic on the website.

Please let me know if you still need assistance. :)

Kind regards,

Annie