Hello, I use Origin 7G and Windows XP. I tried to fit some complex data and suddenly the number of Iterations was increased to some strange value. Afterwards I was not able to fit any curve with a self-defined function. Not even y = ax+b was possible. I always get "Error 28037. Likely caused by errors in user defined formula or poor parameter initialization" It's definetly not a problem of the function since easy functions don't work as well and it is not a problem that the starting values are too far off. I take the same functions as before the problem and it does not work.