You can reserve Vienna - Copenhagen easily directly with the operating traincompanies in your case i would use bahn.de from Vienna till Hamburg (if you are useing daytrains) or oebb.at if useing the direct Nighttrain :)  See my post how to get reservations with short manuals for DB and ÖBB :)

For the reservation from Hamburg to Copenhagen i recommend to use the webiste of  DSB Udland by B-Europe with these website you save the service fee of Interrail :)

 

I just tried it, however I was not able to reserve for multiple people because it said that the reservation is not available out of some reason (DB Website). On the OBB Website, there are no offers for Munich-Hamburg and Hamburg- Copenhagen. I would travel on the 5th of July with 4 friends at 5:30 AM from the Vienna HBF. 

Have no problem to reserve via Bahn.de seats from Vienna via Nürnberg to Hamburg price is 4€ per seat/Journey 

And DSB Udland reservation via B-Europe for 30 danish Krones per seat/train 

I'm not sure if it's a very good idea to go on a journey with a 6-minute connection in Salzburg and a 9-minute connection in München Hbf. I'm not even sure if 9 minutes in München Hbf is sufficient to get from the side station hall to the main station hall (and remember there is no tunnel in München Hbf).

Also, it's generally not a good idea to use the rail planner app for reliable planning. Please check the DB or ÖBB website or planner.

I am sorry to hear you have been experiencing issues when making reservations for you and your friends. This is a bug we are aware of and are trying to fix. As a temporary workaround, you will need to go to the '’My Trips & Travelers section, edit your travelers and add the missing data again. In case you still experience issues, please contact our Customer Support team directly via one of the support options listed here: https://www.interrail.eu/nl/support

However we all have a Mobile Pass and do not have an Issuer and a Pass Cover number.

Is there any other way?

Book reservations via the Deutsche Bahn website or ÖBB website or go to the DB or ÖBB ticket counter.

I have the same problem, but I need an international seat reservation from Germany to France with a TGV, which I cannot book online somewhere else (the service of the french website doesn’t work for me). My friends also have mobile passes and no pass cover number. I need a solution as soon as possible pls :) 

Assuming this is a TGV that starts in Germany and you are in Germany: go to a DB ticket office or call DB at 030 2970 (available 24/7). If you order via telephone, DB can either send the reservation to you (for EUR 5,90) or you can collect it at a DB ticket machine (“Bahn-Tix”).

Unfortunately, I’m in Austria at the moment. Do you think it is somehow possible to collect it from there (Vienna) Thanks :) 

Then try an ÖBB ticket office. But you cannot order from DB and then collect from ÖBB.

But since you will be travelling to Germany (you wrote it is a TGV from Germany), you can still call DB at +49 30 2970 and collect your reservation from a DB ticket machine whenever you get back to Germany.

Yeh we start in Austria and take the TGV in Germany. Ok I try to call the DB office tomorrow, but not sure if you can book from the ÖBB the TGV, lets try. Thanks you :) 

The OBB Staff is able to do it but it depends from Person to Person and which booking platform the Ticketdesk use Austrian rail use since almost 2 years a easier system (looks like the homepage of ÖBB) and depending on the experience of the person in front of it depends if they can do it or not. ) Interrail and Eurail reservations aren´t well trained especially for foreign countries.


Off Topic:
I have access to the German booking system and with all the changes each year it takes for me almost 2 months to get used to it. In Germany few Rail travel agency united to share there knowledge to each member (other travel agency) experienced german Rail travel agency´s

It a shame that the companies doesn´t teach the staff enough and make it harder and harder to reserve trains. Had once a Austrian Conductor that doesn´t know the EuropeanEastPass (like Eurailpass but just for Czech Republic,Hungary,Austria,Slovakia) and i had to discuss with them for more than 30minutes that this pass is a valid ticket.