When I only tested the 2 case classes you provided, I didn't receive the error and my test ran as expected.

Im not the biggest expert on uPickles error messages, but I think it might be some additional dependency in the file interfering with the ReadWriter.

The error text :

/* ambiguous: both method ArrayReader in trait Readers and method join in object ReadWriter match type upickle.default.Reader[
[error]     |      Array[
[error]     |        de.qno.tournamentadmin.lichess.LichessInternalDataTypes.LichessDuelInterna]
[error]     |    ] */

Would give the idea that you might have implemented/included an ArrayReader in the file that attempts to implement the Reader for the Array[LichessDuelInterna].

Maybe you import too many parts from the upickle.default package?

Unfortunately with the limited code sample I cannot say for sure, but I hope this helped.