Verified my guess that the problem is caused by my relations. If I remove them the error doesn't occur. Guessing by the error-message I thought it might be related to the number of ids specified for a relation, but specifying three ids didn't help. Neither did removing all but one relations. Triple-checked the documentation and as far as I can see I do everything exactly as documented?
After reducing the problem description even further I found out that everything works fine if I specify just one relation "in_same_route":
Is there some undocumented requirement in order to use "in_sequence"?