Small update: I was able to get the Paris-Colmar train to show up in the B-Europe website. I am wondering whether it finds no Paris-Bayeux trains, because they actually don’t require a reservation. I was very surprised that they would in the first place. Maybe Eurail and DBahn are incorrect and just say that for all French trains?
But my other TGV (Frankfurt-Paris) is a no go on B-Europe, because I can’t enter Frankfurt as a starting point. I am currently trying to book this leg via the Eurail website.
Paris - Bayeux is on one of the regional train lines for which the Normandy region now wants to have reservations for journeys from/to Paris on Fri/Sat/Sun, see this page. They are not available online but only at the station, maybe only in that region. The staff seem to be quite relaxed about it so don't worry too much.
When you call SNCF, they say to press #85 for English in French (!). After that, it's all English. Not sure if they can book these regional trains to Bayeux though. If you book them this way, do let us know. Otherwise, just go to a Paris ticket office when you arrive in Paris and ask there.
@rvdborgt, thanks for your information. I did use your #85 trick originally, and everything was in English, except the last message before the call termination, which was in rapid French. I do speak French pretty well, but it was beyond my ability to comprehend on the phone.
Your tip about the Normandy trains is very helpful. I suppose I will try to book reservations on site, which I don’t think will be an issue.
Finally, I was able to get my legs to show up in the Eurail website. As you have said Bayeux legs never did, but Paris-Colmar showed up. And I think Frankfurt-Paris was booked. I considered buying separate full price tickets, but it was cheaper to change routes and go through Mannheim. Eurail did allow reservations on that route. If I pushed out my date to July, Frankfurt-Paris was available, which is why I assume that the passholder spaces are booked at this point. The error messages do not allow you to differentiate between the conditions.
So I guess I’ve got as much booked as I can, and I won’t have to cancel the pass after all. Thanks very much for your help @rvdborgt. In the end the Eurail website was able to book most reservations as advertised, but the cryptic error messages and regional trains that could not be booked at all made the process so confusing that I almost gave up. I hope someone else will benefit from my experience.
In France only the TGVs (high speed trains) need a reservation, the regional trains are free with an Interrail pass.
If the reservations are the problem, you can also use the option in the app “no seat reservations required”. The journeys showed take longer obviously, but no reservation means you can do that as a last minute solution.
I found Paris-Bayeux trains in the app, not sure why it didn't work for you, but..
Lastly I noticed that for a specific day there was not a single train departing from my favorite station in France. Super weird. And after a bit of research, I found out that the trains where cancelled that day for construction works on that line.