The problem lies as I can understand in the "ports", where i've defined a Vector Xi_outflow[nXi]. In my case nXi is 2.
The flattening is functioning perfect. And i can simulate the model using Dymola.

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Transalation    17:25:22        line 0:0-0:0    Error building simulator. Buildlog: Syntaxfehler.
gcc  -O3 -falign-functions -msse2 -mfpmath=sse   -I"C:/OpenModelica1.8.0//include/omc" -I. -L"C:/Dokumente und Einstellungen/lvfkif0/Desktop"   -c -o test.o test.c
test.c: In function 'residualFunc18':
test.c:1365: error: '$Pfixedambient1$Pport$PXi_outflow$lB1$c1$rB' undeclared (first use in this function)
test.c:1365: error: (Each undeclared identifier is reported only once
test.c:1365: error: for each function it appears in.)
test.c:1365: error: '$Pvolumeflow1$Pport_a$lB1$rB$PXi_outflow$lB1$rB' undeclared (first use in this function)
test.c:1365: error: '$Pfixedambient2$Pport$PXi_outflow$lB1$c1$rB' undeclared (first use in this function)
test.c:1365: error: '$Pvolumeflow1$Pport_b$lB1$rB$PXi_outflow$lB1$rB' undeclared (first use in this function)
\MinGW\bin\mingw32-make: *** [test.o] Error 1

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I can't upload the files because of restrictions. I would need a email to send the file. Thank you!

Cheers,
Filip