I run into the same issue almost every day when creating new apps. I don’t have any official confirmation, but I can share what I’ve deduced from my observations. I believe, the error is rooted in the cardinality of the involved tables. If you try to combine two tables whose relationship is of type n:m, this error occurs. Celonis typically does not allow for direct n:m relationships, but if you combine three tables with the following relationships, you end up with the same Situation:
lets say you have 3 tables A,B and C. A -> C (1:n), A -> B (1:n). Trying to inlcude table B and C in one Analysis results in said error for me, especially when table C and B use a different set of keys. This situation artificially creates a n:m relationship.
This error also occurs when your case table is on the “wrong” side of a 1:n relationship, such that one row can be related to multiple rows of the case table. This actually makes sense if you think about it from a process mining perspective and it would also explain why keeping your case table on the left side of the join solves the problem.