You're not the only one seeing problem like this. I can't really help you but is this on iPhone with version 23.1.0? (Go to More, scroll to the bottom.) I'm asking because it seems to be the iPhone version that has problems.

In any case, go to More > Settings, scroll to the bottom and enable error logging, so customer service hopefully has logs to look at.

Thanks for your reply. Yes that is the version I have installed. I already enable the logging. 

The question is still what can I do during ticket inspection. What is our workaround? We are traveling again this Tuesday 26 apr in the morning and also in evening. Will we face the same issue? This is so bad and troublesome.!! We have kids travelling with us and what if we are removed from Train or got fined. This problem could simply destroy our trip.  At least now I think I will never buy interrail anymore anymore in the future. Damage to the trip plan  is so high. This expensive pass should help the travel not creating more headaches. This is definitely blocking issue  and not except-able. It is invalid tickets during traveling . So they must fix it immediately. 

And now nobody from customer service response to me yet. 

When you try to add a journey to your pass, is a ravel day activated? You can see that in My Pass. There, you can see the activated travel days If you select the latest one, does it mention the date of the latest travel day, even if the app doesn't generate the ticket?

If so, then you can still show that a travel day has been activated.

You can indeed test this. You can (try to) activate a future travel day by adding a journey on that day. You can also cancel a future travel day by selecting it in My Pass and then tapping on Cancel travel day. This is possible until midnight CET (or now CEST) before the travel day starts.

@Nanja, can you confirm that development is working on this bug?

Thank you for your reply. Yes, the travel has been activated even though the train and Qr code is not correct. I shown that activated day and added train to conductor too. Because his device detected that ticket is from the previous day. After couple minutes of nothing we can do he walked away. But some conductors are very strict so I may not this lucky again.

I tested it and that’s ok for now. I just really hope that problem will not happen again.

Hi Nat, I am trying to locate your message to Customer Support but I can't find it on the email address you used for your account. Did you perhaps use another email? If so please provide me with that one via a private message. 
rvdborgt, I have not seen such case before that a previous travel day is shown, but I will pass on this particular case to our development team. 

Just sent you my email. I am travelling in about an hour and I have the ticket from the morning train generated only. Please help.

What is it that does not work now? Can you add a journey to My Trip? Can you add it to My Pass? Can you show the ticket?

At least you can show that you have an active travel day for 26 April. That should normally be enough.

To be more clear I activated travel day today for the train ticket this morning and the generated ticket was ok. Now there is another train tonight that I have to take. It is not working anymore. Only the train in the morning is shown for the ticket. It is added to My trip but QR code from the morning train.

I'm not sure what you mean here. What happens if you are in My Trip and try to add the evening train to My Pass (with the switch next to the journey)? Is there an error? If so, please post a screenshot.

There is no error if I am adding the train into My trip. Train was added and it seems added ok. But when I click on Show ticket in My pass, the ticket from the first train (morning train) is always shown instead of the current one. Meaning I never be able to show the ticket the second train (evening train). This is unacceptable in point of view.

Indeed it is ok so far. I can show that my pass activated for that travel day. But it really make me nervous and not feeling good every time they check the ticket.

I fully understand. There are quite some problems with the iPhone app currently so I hope development is working on them. Do check for app updates regularly.