Another things to try: clear the browser cache and the cookies for eurail.com.
Book on the Belgian railways website. Price is the same and there's a 4€ booking fee per order (Eurail charges an extra 2€ per person and seat so that's the same in this case). You do need a pass cover number to book there, which by default is only on the paper pass cover.
To get a pass cover number for a mobile pass, you need to contact Customer Support through this form:
https://eurail.zendesk.com/hc/en-001/requests/new
Customer Support is currently overloaded with requests so you will probably have to be patient to get help. Please let Customer Support know what date your trip will start so that they can prioritise your request correctly.