After some more testing, the problem occurs on both machines, just the handling is slightly different:

On Windows 10 built-in compiler the error occurs directly at this line: uvel = C_U(c,t0);

On RedHat system compiler that line is executed but when I try do anything with this value (calculations, output using Message("u %g", uvel); or saving C_UDMI(c,t0,0)=uvel; ) fluent throws an error. I assume uvel just contains a "garbage" value after using C_U(c,t0); however I haven't found a way to check. Outputting other known values or saving to UDMI using the syntax above works fine.