PG,
I see a very big mistake you made in that model. I'm sorry I didn't notice it before, but you are using the FCN block incorrectly.
First of all, instead of a function block, you should use a gain block for the 1/R and 1/C multiplications. That is more straightforward, and I just automatically assumed that that's what you did.
If you prefer to use the FNC blocks, it is certainly no problem, but then you need to change the formulas to u(1)/R and u(1)/C inside. This is because this block is a general arbitrary function block and you can have any number of inputs. In this case you have 1 input to each block, but that input is labeled as u(1).