Any time I try to make a reservation through the bahn.de website it will not allow me to make a seat reservation. I only need the reservation because I have a Eurail pass. But, I want to secure our seats since we will be travelling in the middle of August. The same is true trying to go from Interlaken to Brugge.
Just buy them when you get to Germany. The only reason to buy tickets (with or without reservations) early is to take advantage of discounts. You don't need to secure a reservation this early.
Most travel guides and information sources say that space on trains for Eurail pass holders can be limited and that reservations in August could be needed weeks in advance. Because Munich -> Interlaken and Interlaken -> Brugge are long routes there are only a few trains and connections we can make in order to get to our lodging on time to check in.
The space on those trains wouldn't be any more limited for Eurail pass holders than for anybody else. Eurail pass travelers aren't different than anybody else except for the type of ticket they hold. You don't need reservations unless you are traveling on a popular route during rush hours, or at a time when the universities are on holiday which is not the middle of August.
The Bahn does not make reservations-only online for routes with one terminus outside Germany. However, with sufficient advance purchase, you can probably get Savings Fare tickets for less than the cost of a rail pass day, and you can get reservations with those tickets. In France, TGVs require reservation and French Rail limits the number of reservations for passholder (another caveat for rail passes). If I remember, that is also true of Thalys. For Germany the only trains that require reservations are the ICE Sprinters. They are rare and reservations for passholders are not limited. Even the French TGV running from Stuttgart to Karlsruhe does not require reservations. I don't think Switzerland limits passholder reservations, and looking at the Bahn schedule, none of the trains on most routes to Interlaken have mandatory reservations.
Andrew, you will be fine in getting your reservations when you arrive in Germany. However, you should know about traveling to Brugge from Interlaken. The fastest runs (8.5hr-9.5hr) will route you via Basel/Paris/Brussels/Brugge. From Paris to Brussels, you will be on the Thalys train. The Thalys train has a 39Euro supplemental fee for Eurail passholders and it does limit seats to passholders. Says so on the Eurail website. There is an 11.0hr run that routes via Basel/Frankfurt/Brussels/Brugge. No Thalys train on this run but you will be on the German ICE trains. I believe that they have a 4Euro supplemental fee. You don't get your seat reservations until you pay the supplemental fee. The seats come with the fee.
"and that reservations in August could be needed weeks in advance." I say "Bollocks!" to that. If that's what it says in your guidebook, you need to find a better book.
I found this paragragh on the Eurail website. " Peak season seat allocation "In the high-season (May-September) and during European school holidays we advise you to reserve as far in advance as possible. Some popular high-speed trains have a limited allocation of seats for Eurail travelers, including the French TGV and Thalys trains." I remember years ago, people were posting here complaining that they were stranded in Brussels for days before they could get a Thalys passholder reservations because they were so limited. Of course, they could always pay extra for a Thalys ticket or use slower ICs. The 4€ charge for the ICE is not a supplement, just the regular charge for an optional seat reservation. No one, including a pass holder, is required to have a seat reservation on a normal ICE, but you might have to stand. I did once on a 1/2 hour ICE segment on a major holiday. On that train, every seat was occupied, but only about half were reserved. I could have made a reservation when I bought the ticket, approximately one half hour before train time, and I would have had a seat. The next time I took the same train, there were four other people with me in the coach - plenty of seats.
Another note of caution: I have done this trip and you did not say what type of Eurail pass you had. Going from Munich to Interlaken, you are going to pass through Austria for about 30 km and, voila, that is when the conductor will be checking your ticket. Depending on your pass, it may not cover Austria and, double voila, you will have to pay a supplement! (funny how that happens!) Just know your route and what countries you will be going through to match up with your pass. Also, the TGV in France always requires a supplement and a reservation with a Eurail pass and it is always cheaper to do this in this country before you go. If you wait until you are in Europe, you run the risk of paying the full supplement if the limited Eurail seats are gone (even if the train is only half full).
Our Eurail covers Germany, Austria, Switzerland, and Benelux. Our trip to Interlaken will not go through Austria as I recall. Instead, it goes Munich>Stuttgart or Karlsruhe>Zurich or Basel>Bern>Interlaken. Same with on the way to Brugge. We are avoiding the Thalys and just taking the longer trip up through Germany.