I’m in Italy and I am having the same problem. Were you able to get your reservation?

@Ian Teichelmann @Aroliveras 

There is a limited number of pass holder seats on the Eurostar and on popular departures and during high season those sell out weeks, and sometimes months in advance.

The best place to see the availability of passholder seats on the Eurostar is 

https://www.b-europe.com/EN/Booking/Pass#TravelWish

If you have a Pass Cover Number you can make the reservation online with b-europe.

Otherwise you can call  b-europe and make the reservation. No need of Pass Cover Number.

@Ian Teichelmann 

I checked the availability of passholder seats and right now there are at least 5 seats in 2nd class available on the departure from London to Brussels at 7.04 on the 30th of July.

@Ian Teichelmann @Aroliveras 

You can now generate your own pass cover number here:

https://community.eurail.com/news-and-announcements-39/pass-cover-number-generator-is-live-5653

HI, i was able to book  today by using a windows PC and “Edge” internet browser, it worked perfectly. Before i was using Mac OS and google Chrome. 

@AnnaB Generating the code did not work either. When I was trying to book it did not find my pass.

There seems to be a problem with the Pass Cover Number generator so that some numbers generated don’t work. Customer Supports technical personnel are working on the problem. Try using the Pass Cover Number I11111111 at b-europe and see if that works. @Aroliveras