Did something changed in reporting 5.1.0 in comparison tot 5.0.3? Subreports are not working any more

I have a on premise installation working on 5.0.3 and 5.1.0. In 5.0.3 you could have a parameter with multiple values and in the sub-report working with one value. I want to make a selection and make reports with "datalink". So every feature has only one value passed trough the binding.

In the 5.1.0 release it nags about :

Multi-value Mismatch between Report Parameter parameter1 (MultiValue = False) and Request Parameter parameter1 (a list).

I understand this but is not the way it worked before. The subreport should work on each feature not on the whole set.

So when I change everything to multi-value it nags about:

Error: 0: Unexpected Error: System.Exception: Invalid cast from 'System.String' to 'System.String[]'

I get this because I pass an ID value not a list of values...

The only way it works is when I use single value all the way but then I can generate only 1 report with 1 feature and that is not the way it should work.....