Dear Alfonso,

thank you for your detailed explanation. I selected an existing ROI2ROI analysis and defined my model in the results (second level) tab, but the same error was displayed again: 

Reference to non-existent field 'roiselected2'.
Error in conn_withinbetweenROItest (line 16)
ntargets=CONN_x.Results.xX.roiselected2; %selected target ROIs
(indexes to names below)

So I took a look in the line 16 of the script of conn_wihthinbetweenROItest and there is the following command written: 

ntargets=CONN_x.Results.xX.roiselected2

So I think the script wants to import some information about the ROIs (I selected) out of the results tab. But there is no box in my results (second level) tab to select ROIs (I am using CONN version 20b). I selected my ROIs in the first level tab. I searched and found some screenshots of older CONN versions (like 19) and they showed, that these older CONN versions had a box in the results tab to select ROIs.

Best,
Max