I'm able to reserve seats for those trains on aforementioned dates. Nothing weird going on. Which website did you use?

Best use Deutsche Bahn website - > type in your travel wish in the planner - > seat reservation only and then it should work out.

For alternative ways to book seat reservations, check this community guide. If DB doesn't work, ÖBB or CD might work for you too: 

As such you do not even need to RES for these trains-and in 1st its mostly empty.

@BrendanDB How far in the reservation process did you go on the EC360? I was not able to make a reservation on DB and it stopped at the last step, just before the payment when it says that reservations are not possible to make.

As far as I know, reservations for trains run by ČD, the Czech railway, are possible 30 days in advance. At least that was the case last year when my daughter and I travelled from Praha to Zürich via München. 

I haven’t repeated previous searches but isn’t it a bit soon for reserving seats on regular German services? In general many national operators seem to release reservations for pass holders between 8 and 16 weeks especially where reservations are optional. (Excepting cross border trains of course Par - BCN and Eurostar as examples.)

My experience is that the reservation for Berlin to Prag is possible earlier that the reservation from Prag to München.

I assume that you know that none of these reservations are mandatory. My advice is that you, about a month before your travel, check on DB if the train is expected to have low or high demand. If high demand is expected, then it might be wise to make seat reservations.

Thank you very much for your reassurance. Sounds as though I am trying to be too organised and not wanting to miss out. I will continue to check the various websites and but won’t concern myself until closer to the dates. 

Update on this question.

I am still unable to reserve seats on the (IC143) Amsterdam Centraal - Berlin Hbf / 9:10 - 15:25 on 20/4/2023, this is not only on the Eurail website, but also DB and OBB. Is there something going on this day in Berlin? It’s the only day where a reservation is not possible. 

This is also the case for the (EC360) Prague Hlavni Nadrazi - Munich Hbf / 7:43 - 13:18 on 28/4/2023 service. Both Eurail and DB won’t work, however OBB states the following - 

No Ticket Required

You indicated that you already have a ticket for this route or you are trying to book a ticket for a young child - young children travel free of charge but need to be accompanied.

We don’t already have a ticket for this route and are not trying to book a ticket for a young child.

If you hold a railpass (see your first posting), then you are holding a ticket. The railpass is your ticket. You'd like to do a reservation. 

Reservation for this train is possible 30 days before your travel date (see 2nd answer). This would be the end of march. It's still february. 

Regarding the Amsterdam train I may proceed up to the payment page but of course I don't want to pay. Is there an error when really trying to pay? 

BTW: Don't use DB rather than CD or ÖBB to do a reservation; it nearly doubles the cost.

I can make reservations for this train. I tested DB and CD. Maybe the DB system was in maintenance the past night.

This train can normally be booked 180 days in advance.

With OBB there are 2 ways of getting a reservation only with a Global pass (Seat only or Global pass discount), unlike DBahn which has only one (Seat only) and this can be confusing.

I suspect the message you got is because you tried the Seat only option which for pass holders is not the best option. The best way is when setting up your search you add the pass as a discount option for each passenger. This then offers you all available options with a 100% discount on a paid ticket and you only pay a reservation fee or, if a night train, supplements for your berth.

For more guidance check the reservation guides.

It is a bit more complicated. For pass users you use seat only option on ÖBB for trains with optional reservations. You need the one way ticket option for trains with compulsory reservations and the special fee on trains to Italy. But you can also book only reservations for trains with optional reservations.

Just attempted to book on 20/4 with D Bahn and it offered me a single adult reservation at 4.50 euro.

5,90 € would be the correct price for the journey the OP wants. It seems that you did try 2nd class. 

Correct - the point was simply that bookings were available/